TF2 is a prime example of how a great game can be made even better with time and attention from devs post release.
For a lot of people, including myself, TF2 was the multi-player pack in, and not the main selling point of the collection at the time. I was never a fan of Quake Team Fortress or Team Fortress Classic, and TF2's graphics seemed outlandish and cartoony. It just didn't seem like the game for me. I couldn't have been more wrong.
When I finally gave TF2 a chance, I was amazed at just how good it was. Even without any of the huge updates that have happened to TF2 since release, it was still an excellent shooter. It encouraged teamwork like no other game I had ever played, and while the graphics were silly at first, the game's personality grew on me like no other. Each of the game's 9 classes is so unique that it almost feels like you are playing a different game depending on which one you choose to play as at any different point, and the well balanced maps were fun to play regardless of what class you were running.
Even if Valve never went back and touched TF2 again once it was released, I'd be confident in saying that it would still have quite a few people playing it, however, Valve did essentially the opposite.
It started out with the Medic update, which added in a new game mode, payload, as well as several new items for the Medic class which were unlockable through achievements. Shortly after the Medic, the rest of the classes got updates, then came the item drop system, crafting, hats, ingame bots, training lessons, more game-modes, and maybe most importantly, the Mann-Conomy update, which added a trading system to the game.
As the game stands now, the game comes pre-loaded with about 50 maps, which span several different game modes, and this number can be multiplied exponentially if you factor in the number of custom, user-created maps out there. When I say there is something for everyone in TF2, I mean it. From Capture the Flag, to King of the Hill, to Control Points, to Payload Race, to Deathmatch, to Arena, there is just a ridiculous amount of variety in TF2.
The In-Game economy is nothing short of incredible, and I haven't seen anything like it before, and I don't think anyone will be able to pull it off as well as Valve did with TF2. The game has thousands of items, ranging from common weapons such as the Jarate, which is literally a jar of piss that replaces the Sniper's SMG, and allows him to put out fires and identify enemy spies easier, to promotional items such as the Genuine Dead Cone, which was a Traffic Cone the Pyro could wear on his head, given to those who owned Plants Vs. Zombies on their Steam accounts, to extremely rare Unusual Hats, one of which sold for over $1500 in real money a few weeks ago (as of 7/20/2011).
Oh yeah, and I forgot to mention something, as of June, 2011, TF2 is 100% free. If you managed to get out of giving TF2 a shot for this long, you now have absolutely zero excuses. There are a few things that are limited for F2P players, mainly due to possible exploitation of the ingame economy, namely limited trading rights and limited backpack space, but all it takes is one purchase of about 5 bucks from the ingame store to be, for all intents and purposes, right where the people who bought the game were in terms of in game privileges.
All in all, I can't recommend TF2 enough. The majority of my reviews are fairly objective, but I really can't think of anything bad to say about TF2, there is so much content in this game, and now that its completely free, if you don't at least give this game a shot, you deserve to be kicked in the face, it really is that good, folks.