Gamespy has a
Left 4 Dead 2 Preview
The major points:
New Special Infected:
The Spitter - lobs a glob of damaging spit at survivors, leaving a damaging puddle on the ground. Another Valve counter to shiva-stacking.
New Common Uncommon Infected (Valve needs a better name for this class):
Mud men - move around on all fours, and quickly. Making them hard to spot, especially in the water.
These new common infected will be level specific, which makes sense now that I think about it, so you won't see hazmat zombies in the swamp, or mud-men in the city.
New items:
Adrenaline (can be picked up instead of pills) - allows unlimited melee, and common infected can't slow you down
Ammo Pack (instead of a health kit) - no description in the article
The videos don't show much new, aside from the swamp and shanty town setting. You can catch a glimpse of the spitter's attack if you watch closely, but it's hard to make out.