COD 2 was developed by Infinity Ward, and COD3 was done by Treyarch (Why it sucked) but not to fear the game has been given back to Infinity Ward again, and so far it looks amazing.
A new feature in COD4 is the Class Customization Feature. I'm generally not a fan of class based games (COD3) because you're locked into one type of guy that the developers made. But if I could make my own class, that would be cool. Well don't listen to me talk about watch this video to see what it's all about.