This may just be my opinion, but...
Sure, it was good. But HL2? It doesn't deserve the appraisal everyone is giving it. Sure, it's the best game for the first half of the decade because it didn't have shit graphics and at the time everything in it such as the game mechanics were far above average, but think about it... It was released before the public even knew about the 2nd gen consoles. Sure, good storyline, but it's as linear as every other shooter out there. In my opinion the "game of the decade" cannot be handed to any single game... there's just too many fantastic games out there. For instance, Fallout 3, Portal, they're good for single player games. TF2 and L4D are 2 of the best multilayer games of the decade.
I'm sure most of you would agree with me when I say that HL2 is just too linear. It was so good for it's day almost purely due to it's revolutionary use of a physics engine. If I was forced to pick the best game of the decade that was good compared to the other games released at the same time, sure, HL2. But we have advanced 6 years since then. And in the games industry that is a long way. Hell, even Gears of War is better in some aspects. The storylines are equal in quality and the gameplay is actually more advanced in Gears due to it's use of a cover system and intelligent AIs.
HL2 is also mainly popular because it is almost viral in nature. Because of it's initial success many people had heard about it and told all of their friends it was good. They then told their friends and etc. etc. That means that shitloads of people had played it since it was first released back in 2004. Because so many people have played it there will obviously be more people saying it's a good game, even though nowadays it is nothing special. It's all about the fanbase.