My brother just played the L4D 2 demo (I watched) and here are the 1st impressions.
1. Tanks are not as fast as they used to be.
2. Spitters are hax. There have been cases of 1 spitter wiping a whole team by itself.
3. The new shotgun variant is actually pretty good. Faster reload time.
4. The new assault rifle variant has a large clip and is very accurate but has a long reload time. Good for suppression fire.
5. Melee combat is not only viable, but depending on the weapon, frightenly effective. The machete, axe, and chainsaw kill minor zombies in one hit, allowing for a "Zero" style of play. The bludgeon weapons kill in two. Melee weapons are great for saving pinned team mates (except for the Jockey). Much lower risk of friendly fire.
6. Jockey's (Hunter variant) don't knock down but damage over time while riding your back. Very annoying to take out without friendly fire.
7. Chargers are like a juvenile tank. Basically fight like an angry rhino.
8. Witches now move around. However, if they get lit on fire, they are completly helpless (though it will still provoke a horde if spooked by fire attacks).
