It's a complete game changer in the way narrative was put into something as simplistic as an FPS, not to mention the physics were crazy good and the facial animations were/still are top-tier. Plenty of games have refined the formula since then, and there's a lot of dated stuff in there (the beach is just one big tech demo), but it's revered for being one of the most forward thinking games of its day.
I much prefer the original Half-Life as a gaming experience, but I can see why HL2 was a bigger fuss.
I'm glad I'm not the only person who preferred Half Life 1 over the sequel. Not that HL2 is bad by any stretch of the imagination. Hard to believe Half Life 2 is going to be 10 years old come next November, Half Life 3 is easily 2 or 3 years overdue at this point.
Team Fortress 2 is the Valve game I simply don't get.