The Diablo III Website shows off a trailer from BlizzCon 2011, where the developers at Blizzard discuss the nightmare difficulty in Diablo III, their upcoming action/RPG sequel. In the clip artists Kevin K. Griffith, Nate Bowden, Nick Eberle, and Richie Marella reassure players of the challenge this will represent, which is reflected in the video's title: "Diablo III -- You Will Die. We Promise."
Yes I akin the D3 beta area to Diablo 2 start - Blood Raven. Rarely is it difficult, rarely do you have to use a potion, etc.
That said they did just boost the beta's difficulty. Monsters now have slightly more HP and do a little more damage now. It still isn't hard, but I'm findings mobs are packing together more, which requires a little finesse instead of blind rushing everything, and the elites require quite a bit more to take down and packs of them are fairly common and can be somewhat difficult.
Thankfully since patch 10, the game has become much more 'diablo' flavored, the combat, the monsters, it all 'feels' diablo where it was feeling a bit to much like WoW before that. Looking forward especially to see what they do with the new skill system.
In the end, with confirmed Q2 release now, I'm very much looking forward going from normal to hell mode (and 60) with the same 3 friends I ran through Diablo and Diablo 2.