I have to say, I really enjoy local co-op games with a friend, though it's hard to find one that likes running through whole games like that. It's the team camaraderie thing I like.
For online multiplayer, I like having a friend in the room, it's just more fun that way and it doesn't feel like me vs. the world. Also, I try to avoid teabagging since it's pretty stereotyped as the insult move of the gibbering 12-year old. I just punch the corpse on occasion. It's kinda funny since they often ragdoll across the stage if done right.