you fight L4D infected BUT thats not the only change
at the spawn area there are weapons you can pick up and your loadout it disabled
the weapons at spawn are randomized (meaning no matter what level you could have a chance to spawn in with a minigun or a desert eagle)
players can revive each other
gun damage is increased but not really by much
melee damage (excluding chainsaw) is decreased
health kit time is slower
higher chance of a swarm (but with infected instead of aliens)