Have you ever played versus mode? It's really fun, whether you play with friends or random pugs, and you don't have to be uber l33t to enjoy it.
Also, even though I haven't tried them all, there are new modes. Survival was added as DLC to the first L4D1. Its a mode where endless zombies attack you until you die, the test is how long your group lasts for. This can be pretty frustrating when playing with pugs D:
Again, this really is not a single player experience game, because it's going to be exactly that, kill room of zombies, advance. It's about having coordination and helping our buddies from the special zombies and working together to kill them.
I can't say I've ever played a co-op game that has required actual cooperation as much as this.
Bleh.. Didn't care for part one. Kill a room full of zombies so that you can advance to the next room to kill a group of zombies.. lather.. rinse.. repeat.