Team Fortress 2 is one fantastic game. Valve has managed to design TF2 graphically so well that even it is getting older and older but it still keeps looking good. "Cartoony" graphics also gives the game unique and different look and feel than any other first-person shooter out there.
Gameplay is regular FPS kind but with unique characters and weapons it has that certain feel to it. I admit that somewhere here and there is few balancing issues but they are not that big or noticeable. There is much to play and because Valve is so generous and has provided the modding tools for Source engine, there is always coming more maps to play.
TF2 is also easy game to jump in after longer breaks because it's simple and easy to learn because there isn't much of complicated rules or features.
Probably the biggest thing that makes this game so good and keeps it popular is the constant updates and stuff that Valve gives us. It's very nice to see some developer that cares their games even years after launch.
I bought Team Fortress 2 long after its launch but I still have grown to love it and its developer.