Left 4 Dead 2 isn't exactly like Left 4 Dead 1. It's set during the day, for starters. Oh, and it has new zombies. And new items to help you kill those zombies.
One of the new zombies is the "Spitter", who sits back and lobs spit at you. This spit inflicts damage around the point where it lands, sort of like a mortar. Only, made from zombie spit. As you can imagine, its purpose is to split you up, as a well-placed lob into the middle of a group will force you all to scatter, leaving you vulnerable to the zombies.
Another is an all-new type of zombie: the "common-uncommons", as Valve call them. These are specific to each level, with an example being the "mud men", fast, crawling zombies in the Swamp level that sound a lot like the fast zombies from Half-Life 2.
To help you in your fight this time around, two items - the pills and first aid kits are gone, replaced with ammo packs and adrenaline. Adrenaline works kind of like the old beserk power did in Doom; it stops groups of zombies from slowing you down, and you can go melee crazy, making the most of the sequel's new hand-to-hand weaponry.
Left 4 Dead 2 Exclusive Preview [Gamespy]