supposedly you cannot solo a melee character past normal difficulty. you can solo into nightmare, but you cannot complete it.
A wizard is unbalanced and is the most powerful class in the game, and can solo everything.
I think you have to beat the more difficult areas as a wizard to get access to the items to give to your lower level characters just to get through the game at higher difficulty levels.
Blizzard really dropped the ball this time.
I am in the same boat as you for the moment, I have a level 24 barbarian, and I have not run into the game flaw yet, so I am still enjoying the game, and i only play about 1 or 2 times per week.