I've got a level 40 demon hunter about half way through Act II Nightmare... and wow, they weren't joking when they said "YOU WILL DIE" in response to people who said the beta was too easy. I think I died 3-4 times in all of normal mode, and most of that was just to new boss mechanics. In Nightmare I've died 4-5 times to just a single unique mob. You get a mob with ability combos like Jailer + Desecrator, or Fire Chains + Arcane Enchanted, and it's just instant death. I am still having a blast though.