for many reasons. Valve take great care when creating games, as demonstrated by the directors commentary, and you see that so much effort has gone into every single minutia of the game (which I never got the feeling of in online games such as COD or halo). The classes are increadably balanced, with each having real strengths and equally exploitable weaknesses, making it hard for one player, no matter how good, to domminate an enitire game (though that's not to say it's impossible). In addition to this, they managed to keep each class noticably different, as well as enjoyable, instead of the "classes" on COD, where ultimately you either fell under sniper, or assault.
Next we have the stylised world, which has been given a wonderful painted, cartoony feel that is both refreshing and a joy to behold, and with this they have been able to eccentuate the characters, so each is bith visually, and mentally/emotionally distinct from each-other, and for a game with no direct story to it, to have all nine classes better characterised than the main character from many single player games (Halo, Prototype, Red Faction, etc) you really start to enjoy yourself, and laugh when you get caught out by one of the classes taunt lines.
Now we come to what I consider the strongest aspect of it all. Valve themselves. I've already stated that they put more effort into their games than many companies who have ten times their budget, but they keep on communicating with their fans, and working on the game, meaning that the community will always stay strong, and the game, although it never needs it in my oppinion, always stays fresh with a steady stream of new content, ranging from mear aestetics (like with hats), to actual gameplay changing elements (such as different game modes and new weapons). This, in addition to the strong modding community, allows the game to constantly flourish, with some amazing hillarity arising from it (all those who have played prop hunt will know what I mean).
The game is far from perfect, with the xbox owners (including me) still waiting for the updates, and the game still has exploitable glitches, but it is a masterpiece of online gaming, with it being completely underated by the gaming community as a whole (I just need to ask a few people at my Uni if they've ever played team fortress, and the typical response is "what's that", and these are people who play COD all the time, essentially being magpies being distracted by shiney objects).
I have it both on Xbox and on PC, so I get to see it as what it was, and what it has become, and I can say that it has never wavered from its' core for one second