Unique visuals

HDR is put to good use, without going overboard with it.

Visually Team Fortress 2 looks unique. The developers have gone for the look of early 20th century commercial illustrations. The same style has also been a trend in some recent computer animation films (The Incredibles being the best example). While the visuals may seem "simple" or even "cartoony", it's the exact style the developers have aimed for. Don't be fooled by the simplicity of it all though, Team Fortress 2 actually uses many fairly advanced techniques in the creation of the visuals. HDR lighting and shader work is used throughout, but it's all done carefully and in moderation. Instead of plastering the scene with shiny bits everywhere until it looks like a tech demo masquerading as a game, all that stuff is used only where it's needed to obtain the intended visual look.

I could go on for a long while on character design and shading techniques used, but why not show instead of telling - Valve has produced a great video explaining the work they've done:

There is also more great information on the art and design choices in the developer's commentary track in the game, which doubles as a nice way to check out the featured maps alone without being disturbed by all those rockets and bullets flying in the air during a normal match.