OmegaFenix
The Legendary Troll Hunter
I think, with regards to difficulty, you have to keep in mind that the beta consists of the first few quests on the lowest difficulty. A Difficulty which the devs have said will be "casual". Like Monk said, once u get a few people in Co-op it really starts to scale. I do hope that the difficulty starts ranking up as the story progresses since Nightmare is locked until you finish D3 on Normal as far as I can tell.