To elaborate on what Tonic posted, here's some numbers regarding our mana consumption at level 80.
5644 base mana, our gear has +0 intellect, so we'll sit around 6500-7000 mana in a raid. Replenishment will restore .5% of your mana every second, so 2.5% of your mana as mp5. You gain 169 mp5 in a raid situation....
Crusader Strike is 219 MP on a 6 second cooldown, so -136.9 mp5
Divine Storm is 878 MP on a 10 second cooldown, so -439.0 mp5
Judgement is 351 MP on a 8 second cooldown, so - 219.4 mp5
A Seal every two minutes at 615 MP, so -25.6 mp5
Avenging Wrath every two minutes at 351 MP, so -14.6
You're looking at an average rotation that requires 835.5mp5 to break even. Factor our the 109 mp5 from BoW, we'll need 726 mp5 to break even. Factor in Consecration and Exorcism because they actually do damage now because of Sheath of Light, but they cost 603.8 and 117 mp5 respectively. Oh sweet! They do damage now, but we can't afford to use em.
So, a heavy rotation will require 1556.3 mp5 to sustain. Blizzard is like, "Hey! All your abilities will scale amazingly! But you won't have the mana for em because we're destroying JotW."