And I've never played it.
What I have played is Black Mesa, a massive fan-made upgrade and do-over of the original game, using Valve's shiny new(ish) Source engine.
There were FPS games before Half-Life. One can't describe the history of games without crediting John Carmack's Doom. Doom (and before that, Castle Wolfenstein 3D) laid out many of the basics of the First-Person Shooter.
Doom presented a 3D environment, and the player is essentially an animated camera that moves through that environment. Important information for play is presented around the corners of the image, in what is generally known as a HUD in reference to the "Heads-Up Display" of informational graphics projected on a clear slab in front of the pilot of a combat aircraft.
The basic FPS structure is present here; player avatar and enemies are given an aggregate value of "Health," which is metered or indicated and when used up causes an enemy to be replaced by a corpse or the game to restart at the latest available save point. Player health can be re-stocked from "Medkits," which like refreshes of ammunition and armor, the addition of new equipment, and various (usually temporary) special abilities, appear as objects of a stock (and easily recognizable) type which the Player needs to go to in order to pick up.
Play progresses from Room to Room, Level to Level; Rooms being artificial demarkations (usually but not always architectural) that limit the number of entities and other resources the game engine has to deal with simultaneously, and Levels being completely new data which is read in from permanent storage (usually over some kind of loading screen).
Movement is generally WASD: a set of keys that are more-or-less arranged for convenient forward/back, left/right, with the direction the Player Avatar/camera is pointing controlled by the mouse. Weapons point to the center of the visual field; you aim like you aim the cannon on a fighter plane. Clever players figured out almost instantly that you could in fact fire forwards while walking sideways (holding down Left or Right movement keys) and thus confuse enemy return fire. Thus was born Strafing, and it has had a long an honorable career.
Another stock element introduced back then was the weapons mix; you could generically call the classes Pistol, Assault Rifle, Shotgun, Machine Gun, Sniper Rifle...and add to these different options melee weapons and grenades. Doom offered two melee attacks; the chain saw, and if all else ran out, the Doomguy could defend himself with his fists. The usual trade-off is that the more powerful weapons have less ammunition available, requiring you to strategize which weapons you use at which moments. Add this to strafing, running, sniping, and cover options, and you have a good mix of evolving strategies that keep combat interesting for the length of a game.
The Next Generation
The innovations of Half-Life were not the improved graphics per se. Although they were improved, and this is no slur on Carmack; Doom did extremely clever things mathematically, perceptually, and artistically to get that real-time rendered 3d universe with textures and animation and even (simulated) shadows and lighting effects on those older processors. The Quake engine was the ancestor of the custom code that went into Half-Life, and it permitted polygon characters instead of pre-rendered sprites, real-time lighting effects, and other innovations.
Half-Life2 made huge changes to what became the Source engine, folding in physics originally taken from the Havok plug-in. Graphics upgrades are the name of the game, and one of the big selling points both in the console wars (which are in my opinion largely a forgotten issue as desktop machines outdo them in processing power) and in showcase reels at E3. With HDRI being the most recent flavor-of-the-month; the ability to simulate the high dynamic range of the real world realized both in a higher-contrast image on screen, and rapid changes in gamma as the player moves about.
In any case, Half-Life's major contribution was in story. To have a story, a complex story that unfolded over play. And a story that was presented in-game. This latter part of the lesson is one too few games have learned from. We seem to have reached a point where two disparate philosophies are fighting it out (sometimes within the same game); one is gameplay at all cost (including unrealistic, suspension-of-disbelief breaking, and outright invasive elements in order to provide more control of the gaming element itself), and the other is story, but with an unfortunate twist.
I can't lay the fault at the feet of improved graphics and a market method (the demos and the reliance by most advertisement and game criticism media on eye candy for short attention spans) that make fancy graphics the cart dragging along the unfortunate horse of the game itself into a shiny mess of motion-blurred water-spotted HDRI-enhanced lens flares. Whatever the cause, the game itself plays like the advertisement; player control and even player input is sacrificed for the sake of getting the perfect animated image.
So for better or for (mostly) worse, the cutscene has come back. Whereas early games simply couldn't afford to do more than flash up a page of text explaining who you were and why you should be shooting everything in front of you, modern games treat you to up to twenty minutes of brilliantly choreographed, voice-acted, animated, directed (with all sorts of fancy camera tricks) and lit sequences of, well -- to paraphrase one reviewer, "...an exciting game I wish I was actually playing."
It can be done. Mass Effect, for instance, plays in third person and moves "in" to standard shots for the dialog sections -- mostly two-shots in medium, but there are some action shots. This works I think because you are still an active player; you have switched from WASD keys to Dialog Wheel, and the change in shot is acceptable to go with the change of play style.
Contrast with all the Tomb Raider games produced by Crystal Dynamics; as a general rule, if Lara is in anything other than the standard third person, then you are being a passive observer. The exception -- the Quick Time Events -- do not help. If anything, QTEs make you not even pay attention to the carefully arranged movie and look only for the display of which button you are supposed to mash next (to finish the QTE and get back to playing the game).
(And no, this is not interactivity. Better to call it "reactivity"; if there is only one button that progresses the game, then there is no actual choice being made, even the most trivial such as whether to use the pistol or use the rifle at some random moment of combat, or whether to walk down the right side or the left side of a wide corridor. And choice is what separates playing a game from watching a movie.)
As with Half-Life 2, the story unfolds in front of your eyes. Unlike Half-Life 2, there are not even lectures given by other characters. Well, only very short ones. You pretty much have to figure out what is going on by yourself. And there is an additional element of player choice here; you can figure out just enough to keep going and surviving, or you can listen carefully and look for subtle clues and pick up additional and far more mysterious (and threatening!) levels of story below the surface.
Valve themselves ported the original Half-Life to the new Source engine, but it was just that; a plain port, without new textures or other additions to leverage new options within the Source engine (such as physics, or real-time lighting effects).
It was left up to a group of fans -- a group that expanded to hundreds of members and who worked at it for over seven years -- to make a version of Half-Life that was the equal in updated graphics and play to Half-Life 2. And more kudos to them because they resisted the urge to remake the game to their taste. It is more like a second draft, with some of that hindsight editing that various authors have found themselves doing when re-issuing an old book that had since turned into the first book of a series.
As in, there are some references that tie it more closely to Half-Life 2...all the way out to a subtle reference to Magnusson (who only appears in Episode 2) and a possible explanation for why he still hasn't "forgiven Gordon for what happened."
It is as I said a second draft. The game is still there, it looks and feels and plays like the same game. But the architecture of Black Mesa has been rationalized a little, some pointless episodes were truncated or removed, play flows a little better, NPCs are much improved in their rendering and in their activities. And there is simply more; more textures, more lighting, larger arenas, more voice acting for the NPCs.
On the negative -- and very likely because it was written by people who had played the original a lot, it is also harder than the original. Significantly.
The basic story is simple (yet enough to hold interest). You are Gordon Freeman, with a doctorate in Theoretical Physics, an interest in physical fitness, and with that and your position at the bottom of the Black Mesa facility's totem pole (being young and new) your job revolves around donning the heavy protective HEV suit and working in the test chamber at Anomalous Materials.
The first episode ("episodes" flow from one to the other with no more interruption than necessary loading time. Essentially, you play in real time, from the moment you enter the facility until the moment the G-Man comes for you at the end) is all introduction. You are free to wander parts of the Black Mesa facility, interacting with the scientists and guards, reading black boards, throwing ash trays -- whatever. In fact, the first five minutes of the game are nothing but a tram ride through the outer parts of the sprawling mostly-underground complex.
These are the parts that feel to me like the heart of Half-Life. The almost indescribable loneliness, as you explore a workplace turned into a shattered ruin with only a few scattered, terrified, and very much temporary survivors. And things are appearing there that wreak their own hideous transformations. There is a profound alienation going on here.
This is the most Survival Horror the game gets. In the earliest phases you have no weapon at all -- perhaps some road flares you pick up. Soon enough you stumble on Gordon Freeman's signature weapon; the crowbar. And this is also an innovation, because it is not just a weapon, but it is also a key; it allows you to access places you could not travel to without it.
Eventually, of course, you find more weapons. But that is largely because they are being brought in from outside. After struggling so long to make it back to the surface, working your way through dripping maintenance tunnels and wrecked offices and elevator shafts with broken elevators, you are nearing the surface when "rescue" efforts show.
And begin shooting at the survivors. You grasp -- guess, really -- that the reaction of the government was to send in experts at covering up. The HECU troops that start piling in from the surface -- pinching you and any other survivors between them and the aliens who are still teleporting in from Xen -- mean to leave no witnesses of any species.
Thus the middle part of the game is a much larger mix of weapons and tactics and an ever-widening picture as you wage a one-man war of survival against the military, with the aliens an equal threat to both of you. Eventually the aliens get the upper hand on the military, and they withdraw to begin pounding the surface with artillery and aircraft.
But you've gone back underground for other reasons. The rip in space is getting larger and threatens the planet and some of the surviving scientists might be the best chance of stopping it. So this is the Act II/III break; you've changed from a selfish survivor to someone who is acting more as a hero.
Which eventually leads you to Xen itself, to stop the invasions at their source. Which sets up the rest of the series. But since the Xen chapters aren't present in the current version of Black Mesa, I'll say no more about them.
The levels are distinct in character (as well as spectacular). As with Half-Life 2, the game makes a point of breaking up the pattern. One section might be close-in fighting, one might be exploration or even (shudder) platforming. And each level and each section of game is distinct in not just looks and not even just play, but in the atmosphere; the chilly cleanliness and relative calm of the labs where Questionable Ethics happen, the hot desert sun of Surface Tension with its wide-open spaces of bare rock and sand the perfect place for the increasing numbers of the military to pound on you, the cubical warren of Office Complex that turns it into a hundred single encounters in close confines, etc.
There is a multiplicity of weapons and options, even more so then in Half-Life 2, and many have dual modes; rifles that fire grenades, satchel charges that can be thrown with one click and detonated with another. Too much wealth, in fact; in the height of the fight against the HECU one is sometimes getting killed through being distracted by wondering which particular mix of weapons and strategies will work best. (I managed to kill myself with trip mines multiple times by trying to get clever with them).
New wrinkles continue to unfold in the story. The combat and exploration is enough to stay interested in any case, but there are slow revelations throughout, strongly showing that neither Xen nor the Resonance Cascade itself were entirely unknown or unexpected.
Interaction is simplistic. Since you don't speak, all you can really do is press the "interact" key when near a character and see if they will talk to you. But this still feels better than some games. There aren't characters designated to inexplicably go into lecture mode and hold you captive while they explain to you who you are and why you are there and why you should care about the invading monsters. Instead the other characters all appear to have inner lives; they are going on about their own business and only glancingly and tangentially end up explaining anything to you.
One oddity is crouch-jump. There are perhaps two places in the game where normal jump (aka hit the space bar) is actually enough. Everywhere else, it is necessary to do a super jump by hitting the crouch command and immediately following with the jump key. So for all practical purposes standard jump could have been used instead; it is a wasteful extra key operation, with the only tangible benefit being certain YouTube videos.
Black Mesa also successfully reproduces something else about the Half-Life experience; and that is the difficulty of detaching from a ladder. You have to be positioned perfectly in order to move gracefully off a ladder. Anything less and instead of moving on to a catwalk, your command causes Gordon Freeman to hurl himself into empty space and die impaled on broken crates far below. It makes every single use of a ladder in the game a mini platforming game in itself.
I have to admit to save-scrumming through parts of the game. Not so much the combat parts (although the lobby fight is incredibly tough), but for the platforming sections. The Box-Smashing Room in particular, but also the cliff-side climbing (where you have to crouch-jump at an angle to a narrow target and immediately perform a second action to keep from falling to your doom.) Fortunately the game allows saving a game at any arbitrary point.
What should have been learned
An immersive experience is possible. A game doesn't have to mimic film, with the downside of turning the player into a passive viewer, in order to be emotionally involving. And it doesn't need these tools to communicate story; a complex and nuanced story can be put across in the same way most of us experience the stories we encounter in our daily lives; not as a presentation but as a collection of observations and small interactions.
The Portal series does this. But there's no reason it can't also be accomplished in third person. Mass Effect and Deadspace and Bioshock also rarely feel it necessary to cut away to a cutscene in order to communicate story beats.
Related to this is trusting the audience. You don't have to spell it out. Also, you don't have to force them to absorb the story. If all the want to know is "Aliens bad, Army bad, just don't shoot the guys in blue or the people in lab coats. Unless it's fun" then they can go right ahead and play.
You can constrain the path and create a linear narrative without making it blatantly obvious you are doing so. And, hell -- on of Half-Life's chapters is actually called "On A Rail!" I'm not entirely sure what makes the path feel open. There are alternate routes. There are dead ends that go for long enough you aren't just being told there's only one door. There are rooms large enough to move around in and explore for a while.
One way to make this work is lock-based or hub-based environments; there will be a key (an action, an item, a puzzle, an interaction with an NPC, a fight that must be won) before the door to the next level can be opened. But before that, you can explore and exploit and otherwise solve the level in whatever order you wish.
Paths are less annoying when the environment makes them logical. A narrow trail up a cliffside is understandable as being only one open path to proceed. A wild wilderness area is a lot less convincing, and the waist-high weeds that conveniently prevent you from moving off the designer's intended path are at best felt as an imposition -- at worst, they are immersion breaking (as yet, few games have managed to explain why your barbarian character with a sword that can cleave an iron-scaled dragon in two is stymied in her path by a flimsy wooden door).
Designing logical environments works. Players aren't all architects or civil engineers but most of us do have an innate sense of when an environment is functional and laid out in the way human spaces are laid out, and we lose belief in a game universe and lose some of that important emotional connection when presented with scenery that was clearly arranged only for the benefit of the monsters (or for creating a specific movement path).
When all is said and done, it is a good game, and via Black Mesa completely playable today without even having to make excuses about "the charms of a retro experience" or anything.