From: IGN.com"
IGN: What about Team Fortress 2? Stylistically it's certainly a very different departure for Valve...
Newell: In the multiplayer arena we already have the gritty realism provided by Day of Defeat and Counter-Strike, so for Team Fortress 2 we wanted to do something different and not just a remake of games that people already have. We made the focus of the game the different classes of the soldiers, so each troop type has a different role that is complementary to the next. What's clever is that if a player learns one role as well as how other people play their roles they become much better player. That's because the game is completely dependent on teamwork.

We also wanted to make each class readable, even from a distance. If you see a player in the distance in Counter-Strike it's not particularly important to know a lot about that character. In Team Fortress 2 knowing the difference between a sniper and the pyro guy is incredibly important. That's we decided to opt for very stylized graphics, because the easiest thing to read from a distance is the silhouette of an object. The outline of the different classes therefore varies a lot, so the player can recognize the type of enemy they're up against even when they're only a couple of pixels high. It's the same with the way they move: the scout, for example, doesn't move realistically but his exaggerated strides make it much easier for gamers to pick out.
IGN: Do you think TF2 will become as popular as Counter-Strike?
Newell: You can never tell. We work hard and give the product to our customers. The rest is up to them."