I'm not in bleeding edge content so when I raid easymode I do it for free loots and to do as much damage as possible, preferably end up #1 on dps. Thats wherein the challenge lies for me, and with unbalanced classes the dps game becomes utter shit. My first MoP char was a shadow priest, which is fucking horribad at DPS, dead last on most (if not all?) sims and with a very simple damage priority at that. I played that char one week at 90 before I switched because there was no way in flying spaghettisauce that I could compete with that class. DPS classes should ideally be as close as possible on standard types of encounters, if there is such a thing. If so the dps comes down to skill and becomes interesting, if the classes differ more than a couple % it will start to show on the meters.
PvP balance has caused major pain for the PvE game (and vice versa, I'd assume) ever since they started balancing classes for PvP, I really think Blizzard should swallow their pride on this and make the PvE & PvP balance completely separate. It already plays out so different that they're almost separate games at this point, so I really don't see the harm in doing it.