HL1 established that a FPS could have a plot and still have fantastic game play. HL2 basically established that all other FPS's and most other games where stupid and that you can have a real story, real characters, and a truly cinematic experience from a game.
It might not be your kind of story, it might not be your kind of game play. But if you look back at game play of almost any genre before 2005 and after lets say 2007 (games that started or were early into development after HL2's release). HL2 is directly responsible for the dramatic increase in game quality and basically set expectations so high that garbage that used to come out can't see the light of day now. I mean most games 40-60 now were better than most 80+ games back then.
Now if HL2 alone was released today. It would be considered a little short and graphically distracting, while being a little too on rails even if it pushes the "movie feel" of it. I guess it would be 80-85 rated game. Which is still really good. No game is going to hold up for 10+ years later, but this one better than most.