Judging from a few gameplay videos out there and some articles, I don't think he's a tank. I think he's a Charger, who's main purpose is to punish closet campers and piledrive people. Not sure how that works with 1 beefy arm, but I'm not gonna argue with a guy suffering from trogdorism.
They're also talking about porting all the L4D1 content into 2 so you don't have to switch games to play the old levels.
New AI director will also change weather effects and which paths are open to you. So if you're doing poorly they might give you a straight line, and if you're doing well it might get a bit more complicated.