Really, the game is not going to show much difficulty till you get into the next difficulties. I already was hearing stories in General Chat from Nightmare players that were struggling in Act 2.4 person COOP definitely seems easier than it should be.
This is not even going into the fact they added a whole new difficulty, Inferno, which is supposed to be so hard you can't even handle it till you reach cap and get some good gear. That will be interesting to play with friends.