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.
PvE balance matters because if it didn't, everyone would run a 25 man druid raid comp.
Because of procs, rppm and more right now, there is no rotation. There are priorities that you need to follow, like make sure to refresh dots when you have a haste buff or only use Wildstrike when you will have one up for Collossus Smash, but there is no rotation anymore. You cannot predict exactly what will happen and need to be capable of reacting to procs and effects that occur