I don't mind dungeons being hard, but what I really liked about LK dungeons was the time needed to get through one. Don't get me wrong on this one, even a damn freaking hard dungeon doesn't need to take well over an hour. If your group/gear sucks and you wipe at the first trash pack is fine. But having to go through endless wings of a dungeon and/or additionally masses of trash isn't fun, imho.
From what I've heard/seen so far from cataclysm dungeons is that they do take quite longer. But that might be because of players lacking itemization before going in, I hope.
Anyway, I really hope it will be still possible to do something in cataclysm without having to play 1-2hours at once or raiding in general. I have no time for that.
PS: I don't want well fare epics or whatever, just some reason to log in, because I like that game ... Getting emblems for heroics that could be done in 15-30minutes was a reason for me in LK, because it was fun, fast and after some weeks you could get something out of it. You were still far behind active players, though and that's ok.