I think, TBH, ramping up the difficulty online's a bad idea; the Director works so well, and on high difficulty levels it reacts to you all still being alive by throwing Tank after Tank at you. That game I had last night with HoriZon and ecureuil was a nightmare for it.Wimp! ;P
I'm probably a bit perverse but being completely overwhelmed by the horde and / or boss zombies is what makes L4D enjoyable for me. You have to keep moving to stand a chance and teamplay becomes essential for survival. Tanks are really fecking hard to kill but again, with good players working as a team, it's quite possible, and very satisfying, to bring them down. Napalm is really useful for this. Chuck and run run run.
L4D is the first game I've played where dying is fun.