Finished first playthrough a bit ago. Thought it wasn't tuned very well for a party of four on normal -- we'd have had to make pretty egregious mistakes to be in any real danger even though we were of proper level. We only had a few deaths in Act 3/4 and they probably wouldn't have happened if we were actually being threatened, but it was just lazy stuff (ie, I died with the final boss at about 1% because I didn't feel like bothering to get out of the way at that point in the encounter and other deaths were similarly to weird shit where people didn't make easy plays out of apathy). Imagine the higher difficulties will be really fun because all of the end act bosses and a couple of the other ones had fights that had mechanics that seemed like they would be interesting if damage was high enough to actually punish mistakes.
Some tips for people who haven't gotten far yet: the base interface makes a few decisions that are intended to make it easier if you're completely stupid or something but that greatly inhibit understanding and playing the game well. Hit Esc, go into Options, Gameplay, and turn Elective Mode(able to bind whichever 6 skills you want to the 6 slots) and Advanced Tooltips(descriptions of damage/effect of abilities that actually mean something) on.