WW2 weapons, but I am intrigued by the flame thrower and more vehicles and mounted weapons in multiplayer.
When I go back to 2, I miss the perks and the NPC assists (especially the helicopter).
Edit: CoD:WaW will probably have some of this, but no heli of course. Maybe tanks assists, artillery, or some other cool thing. Kamekazi?
I would cite regulation, but I know you will simply ignore it.