One definite criticism I have is that I do not like the fact that progressing past a certain point is tied to expert mode. Powerful drops from bosses is fine, but the ability to even fight a boss being locked behind difficulty doesn't sit right with me.
So, I found a way to progress further, and managed to beat Erazor. I still need to beat Nihilus, but on the upside, I managed to beat Araghur without getting hit once. Still working on Abbadon, but no dice so far.