Call of Duty: World at War

Graphics have improved along with sounds

Multiplayer has been enhanced with new perks and weapons and vehicles

Online co-op campaign (unseen in previous games)

New "zombie mode" in which you fight hordes of "Nazi zombies" buying weapons to help you fight for survival (players 1-4)

Deeper, richer but seemingly shorter story than previous game.

Personally I prefer Call of Duty: World at War over Call of Duty 4: Modern Warfare mostly because of the enhanced online play, new weapons, more ranks, new perks and added vehicles (unseen in previous game) spice up the online experience

Guns sounds more like weapons, explosives feel more powerful and the experience feels like you've been dropped straight into the middle of the second World War an experience previous games in the franchise have not had, also clever voice overs from well known characters has added to an already deep story and this is a game I can promise you will keep you hooked for hours, days and maybe even weeks.