I'll start things off with this report from the MoP beta discussion thread (context is enhancement aoe mana cost):
Fire Nova is almost certainly bugged, or else Mental Quickness has an incorrect tooltip. MQ is supposed to reduce the cost of "your instant beneficial, damaging, and totem spells" by 75%. Fire Nova's current cost in Enhancement spec is 5140. When you switch to Elemental or Resto and page back to the inactive Enhancement spell list, however, Fire Nova is still listed at 5140, instead of the 20560 it would be if it was previously under the effect of Mental Quickness. (To see this for yourself, switch from Enhancement to Resto or Elemental and check Earth Shock's and Feral Spirit's mana costs before and after.) This means that FN was never under the effect of MQ in the first place, despite meeting all the criteria for it.