Bah, I just lost my lvl 56 HC barb. Act 1 of Hell monster power 10. Just a random pack of elites. I killed 3 of the 4 but then suddenly my health stopped regenerating and that was it. Sucks, but oh well. I guess I should have backed off the monster power a bit.
Yeah I never used monster power and cruised right into inferno with pretty much no problems. But if you want to challenge yourself MP is the way to go.