Re: What is the real difference between the difficutlies?

I know it doesn't really answer your question (because I don't know the difference, but I play on nightmare for the meta achievement so I can't be changing it), but I know enemies have more health and seem to do more damage as well. As for the respawn rate, yep. And it's worse than that even. 

Fight your way through rapidly respawning enemies, clear general area, quicksave, die on next group. Game reloads and you load in, still at low health from last fight (which cleared the area as of the save), and are now completely surrounded by mobs that magically respawned with the reload and immediately aggro as soon as you load in, pushing your sh*t in. Save lost. GG, reload sometime earlier. Hope it was recent.