Left 4 Dead 2 will award the avatars of skilled Xbox 360 gamers with frying pans, themed t-shirts and med kits, according to text found buried in the game's demo by Steam forum posters.
Steam poster Fade Guy said he found a list of Avatar Awards in a file that also mentions a commentary mode featuring audio be Gabe Newell for the game.
Here's the full list of Avatar Awards and how you earn them, according to Fade Guy:
Med Kit: Earned by beating all five campaigns. Bull Shifters Shirt: earned by winning 10 games of Versus. A Left 4 Dead 2 shirt: Earned by winning 10 games of Scavenge. Zombie Hand Shirt: Earned by killing 10,000 Infected. Depeche Mode Shirt: Earned by rescuing Gnome Chompski from the Dark Carnival.
I believe this will be only the second game to support Avatar goodies that you can unlock through gameplay.
Almost all of the content in L4D2 uncompressed and viewable [Steam Forums]