I don't think anyone was expecting a sequel so soon, it is actually quite a surprise but here are the new Left 4 Dead 2 E3 details from the recent Microsoft Press conference:
httpvh://www.youtube.com/watch?v=75SFa7VV5X8
Now We can all live out our Evil Dead fantasies and become Ash with the new the Chainsaw!
Left 4 Dead 2 will feature five new campaigns, as opposed to the original's four. Each campaign will have support for four-player co-op, as well as Versus mode and Survival mode. There is one new gameplay mode that is not being revealed at this time. There are four new characters, new weapons, entirely new melee weapons, and new items. On the flip side, there are new zombies, new special zombies, and a bunch of new gameplay mechanics designed to disrupt many of the tactics and exploits that players have discovered in the first game
The game will introduce AI Director 2.0 taking L4D's dynamic gameplay to the next level by giving the Director the ability to procedurally change weather effects, world objects, and pathways in addition to tailoring the enemy population, effects, and sounds to match the players' performance.
RockPaperShotGun Podcast.