Half-Life over the years has shown its age, but still it's an innovative, engaging game experience.
The reason for this is not only the innovative way the story is told compared to other FPS games at the time, but also the fact that Half-Life combines more than one gameplay element at once, and oftentimes they are combined together to give Half-Life its flowing gameplay dynamic.
Half-Life has some really smart level-design. All Health and HEV regenerators are placed where most players would need it and you can save at any point in the game, so there's never a time where the game is at a frustrating level of difficulty.
The game features environmental puzzles where you're required to jump, crawl, and crowbar your way through terrain and obstacles. Although the puzzle elements are not as deep or complex as in games like Legend of Zelda, the variety of weapons and the way you apply them to combat more than makes up for it. This also makes for a very free and flexible approach to dispatching enemies and lets you craft your own battle tactics. The weapons range from an entry-level pistol, to alien weaponry.
In Half-Life, you play as Gordon Freeman, a theoretical physicist at the Black Mesa Research facilities and you are chosen to do some sort of controversial experiment. This goes horribly wrong and, well, all kinds of things start happening that can only be seen to be believed, and you not only see it all, but experience it all, and defeat it all in Half-Life.
Throughout the game you'll have to make your way through numerous locations, not only inside the Black Mesa Research facilities, but also in a sewage treatment plant and a world inside a separate dimension. The theme of the story has to be Science-fiction at its most purest level. One reason for this is, well, your character himself is a scientist, half of your allies are scientists, and most of the game takes place in some sort of science facility. But what happens beyond this is quite literally, out of this world. Unfortunately, this can be seen as generic, but it still is a simple concept, so I guess that is what makes the story setting unique.
Despite its amazing gameplay and presentation, Half-Life is not without its drawbacks. One of which is the monotonous characters and another is its somewhat linear story. Although the linearity is understandable, as sandbox style storylines in 1998 were only at an experimental level, one can't help but feel that the game would benefit from more than one path to the finish line.
In the end one realizes that Half-life isn't just a First-person shooter. It is an Action/Adventure, Mystery Science-Fiction, Platforming, Butt Kicking First-person shooter. It is the one game that set the standard for immersive FPS adventure games to come and established Valve's identity as a symbol of quality game design.