Half-Life was like a magic trick. It was a game you could show to people who weren't gamers and they'd get into it, a gateway drug. A real game (not some glorified puzzle book like Myst) that had the cinematic flair and presentation to suck all sorts of people into it. It was that train ride at the start of the game, casting Gordon Freeman as a dude with an interesting, but ultimately mundane job.
It was how you didn't get a gun in your hand until almost an hour deep into the game. It was the semi-believable lab environment that relied on you looking around for clues and direction in the world, finding vents and trying to worm your way through a building given over to chaos. These ideas are old hat now, but in 1998 they were all groundbreaking.