Ok, read this, it's put my mind slightly at ease and hopefully it will help you guys.
http://www.rockpapershotgun.com/2009/06/01/left-4-dead-2-exclusive-rps-preview/#more-12370
It does basically feel like a great big HUGE bit of DLC, but it's big enough that I can understand a completely new game (I'm not that happy about it but still). It's basically Left 4 Dead, and then some.
That means all the original boss infected (Smoker, Tank, Witch ect) plus new ones, of which two have been announced. One is a kind of mix of the tank and a normal infected, like it's take sterioids but it's only worked down one side. It basically pushes with his huge heman arm (sounds like how a big daddy does) or grabs you and keeps smacking you to the ground until someone rescues you. Theres a wondering witch (only appears in new day light levels) who instead of crying sings an eary song and actually moves around, creeping up on survivors and so on, meaning its a lot harder to avoid her.
There seems to be a little more enthesis on story this time. Not hugely but each campaign leads into each other for instance, and the new survivors have more back story and develope as they move though the campaigns
The maps also sound a lot more open too, theres lots of different directions and secret paths to take, some of which I think the Director may force you to take (puts a vehical in your way or something)
Theres also weird minor stuff like zombies that wear hazard suits (these ones won't be effected by fire) and bullets that set your enemies alight.
Theres also ofcoarse melee weapons, which include a bassball bat, a frypan (What?) a chainsaw and another I've forgotten (I think it may be a shovel). I think you may have less bullets meaning that melee weapons become essential.
This is still really, really weird, like weirder than by dad turning out to be a goldfish. But still...congratulations ValvE! It took you less than a decade to make a game. That quite an achievement.
EDIT: Forth melee weapon is an axe. You can use it to put off zombie limbs and head as they run at you.