Ya but I've been kinda pissy about it cause of Valve's initial promises with Left 4 Dead 1. They claimed the AI would have more control originally (i.e. Gas Station event was going to be decided on big boom based on how well the players were doing, not they shoot gas station and it blows up) and now they have those events in L4D2 (see bridge videos/interviews) and it seems like L4D1 was beta we paid for lol. Or, at the very least, L4D2 shouldn't be priced as a full game cause, to me, it's nothing more than an expansion.
But I see "new campaign, a few new units, and a few new tweaks, but over all same gameplay" as always being an expansion. Just as Westwood and Blizzard games always had it with their main focus games and the expansions of em. But I guess since those were RTSes it's a bit different. I dunno, I'm probably going to get it though cause I <3 Zombies and Valve really does make good games even if they disappoint me in some way. The demo was indeed fun though. The new Special Infected make it even more fun and challenging and the new tweaks to the AI was good.