It just seems that this expansion was one of the more brutal curves in terms of the rating needed, possibly because its only over 5 levels instead of 10?
Well the whole reason they do it is to do a total gear wipe and leave themselves room for new gear. So that's probably exactly what happened.
Yeah, the spell thing has to do with the massive bump to base health / mana you get as you ding through the 80's. Spell costs are calculated off of a % of Base mana (Ie, how much mana you have naked with no buffs). At 80, in icc gear, your "base" mana pool probably covered about 20% of your total max mana. At 83, unless you have been heavily replacing gear, your Base mana pool was probably covering like 50 or 60% of your total mana pool, which caused spells to really eat up your available mana. It balances back out once you are in full heroic / basic raid gear, but in the middle levels it's a real bitch for casters.