In the video the guy says him and his buddies wail away on a boss for an hour? Let's see now. Either these guys have really crap items or the boss is super difficult.
In the case of the boss being super difficult then we can think about how many boss monsters we might face in nightmare difficulty, oh let's say 50 bosses over all acts? That means over 50 hours to just beat the bosses? WTF!? Then in theory the difficulty ramps up even more in hell so you might take hundreds of hours just to beat the bosses in hell.
I don't know, I really am beginning to think these people have made a stupid game, and I am feeling rather stupid myself for pre-ordering it