I think Ardith raises some good points to consider.
AoN is not my favorite skill to have been added to the game. I wouldn't want to play a paladin up against an MR barbarian, nor even a hellion or rogue, whose best skills are so easily negated by the barb class in general. The barbarian class, of course, has often been mocked in the past but now is hovering near the top of the 'food chain'. I suspect the particular MR barb causing this immediate consternation also has other things affecting his situation, maximizing his potential.
There is room for the MR barb to be balanced further, as will always be the case with one class or another. Immediately, though, I can think of two classes in particular, albeit rather squishy classes, whom are actually quite well suited to dealing with MR barbs-- when they can live long enough to do so.
