You guys were playing L4D and I wasn't invited?!?!
Well I guess this means we'll just have to get a group together and play more. Oh darn...
EDIT: Ok, played through the demo a few times (Both with and without people) and I must say, massive improvement over the first one! The new Special Infected really change your strategies dynamically (The Spitter, who lobs balls of acid that splash and do area of effect damage are particularly nasty during rushes and crescendos, as it makes it almost impossible to camp. The Charger makes it even harder to split up from your group, as he likes to wait until someone is going a different direction and carry them away. The Jockey is really nasty in combination with the other Special Infected. I once had him jump me and run me into an alarmed car, and then into Spitter goop. Did not end well. There is also the Wandering Witch, that replaces the Witch during the day. She is much harder to startle, but she walks around, potentially surprising the Survivors rather suddenly and unpleasantly). There are also Uncommon Infected, special kinds that show up less frequently than the Common Infected, but much more than the Special ones. These are unique to each level. The one for the level in the demo is a Zombified Riot Police Officer, so he is wearing a bullet proof vest, forcing you to knock him around until you can get a shot off from behind, where he is unarmored. The level design (At least the two chapters in the demo) are much more open world and diverse. Not the strictly linear paths from before, which opens up some options for exploration to find all the sweet new equipment. Along with the classic guns and items from the first game, there are also several new guns (Including an AK47, SCAR, a Desert Eagle that replaces your Akimbo Pistols, etc) as well as an array of melee weapons (I LOVE picking up the frying pan and smacking the crap out of everything. It makes a nice satisfying TONG when it hits a Zombie). There are also two more healing items (An Adrenaline Needle that makes you Melee faster and immune to fatigue for a short while and a Defibrillator that can actually bring a Survivor back to life if you die) as well as a new grenade (Vile of Boomer Goop that attracts the Infected in the same way a Boomer spitting on you does. Great fun to throw at the Tank). All in all, even without the two new game modes (Realistic mode, where you get a lot less ammo and take a lot more damage, and Scavenger Mode where you have 10 minutes to collect as many gas cans as possible), the game is much improved, adding a lot more team work dynamics, and changing up the formula to make it fresh and interesting.
EDIT EDIT: Oh, and I forgot to mention, the AI Director has far more control this time around, even having the ability to change the weather and paths through certain parts of the level.