Me and Deadly_Nightshade joined a modded Left 4 Dead server yesterday - I've never been pushed to my limits like that before..
It was like playing on Impossible compared to Expert. All the special infected were tougher, would hit harder and were faster (Hunters could jump quite far etc) and the normal infected would deal out alot of damage, on top of that they had increased the number of infected, so often you would come across a "rave party" of 100+ just waiting to rush you (and the AI director always saw fit to spawn a new horde while the already huge one was on top of you). I think we had almost 150-200 on screen at one time.
The survivors melee was harder as well luckily, so often the most viable strategy was to stick together and push back the endless hordes.
It was the most fun I've had in L4D yet!