Shadow priest is easy on paper, much like affliction but is a lot deeper than a lot give credit for. Most people do tremendously bad DPS as shadow not only because the class is a bit under tuned, but because they don't play it properly. Dot snap shotting is important for shadow, but it's a little bit different compared to how others who worry about snap shotting dots do it. Also not understanding when to cutoff insanity is a pretty big thing.
Frost mage seems fairly easy in PvE but the king is probably elemental shaman at the moment.
Destruction would be fairly straight forward if it wasn't for rolling immolates with high SP and or crit (more important for crit) and choas bolt reaction within procs etc.