5 new campaigns, 4 new survivers, 3 new infected, melee weapons and a lot more new guns instead of the measly 5 L4D has, lack of variety in weapons always annoyed me in the original, a new grenade type, new ammo types ( incendiary and explosive) 5 new campaigns, new gamemodes. It's a great improvement thb, nothing at all wrong with L4D though, I still like playing it.