This was from IGN
regarding the extras:
The only extra content you'll find is in the form of Achievements left over from the Xbox 360 version of the game. Unlocking achievements by finding dead birds, pieces of metal, special TVs (minus the Xbox 360 models) and beating levels means you'll be able to view pieces of special content via the achievement submenu. These are basically production bits including early animations and concept work. While it seemed pretty strange for these to be left in originally, it's actually nice to be awarded for your work through a game. It may not put any achievement points for all PC gamers to see, but it does add something to the experience.