I said "+ buffs" because combining shammy GoA + pally BoK + upgrades to those buffs + possible new feral druid buffs (and more Agi from new Mark of Wild) + more Agi from Elixir + Lightning Reflexes... well, it's just plain crazy how much that is going to add up to!
Rating -> Percentage is dependent solely on your own level from the information I've gotten. If you want an in-game example to compare it to, think of the way Agi -> Crit changes for melee classes as they level.
Rob, I doubt hunters will see 50% crit, given that only Karazhan+ gear is quoted as being an improvement over Naxx gear, and Naxx gear won't allow 50% crit as is, even before it begins to scale down with level.
BoK + LR is actually multiplicative, so thats 27.5% Agi increase of all the previous 100s Agi's in that buff list.