It's all in the scaling from 81 to 85, yes. Everything prior to that is even more hilariously faceroll easy than ever with the 4.0 talents.

 

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Spell costs are based on Base Mana, which is different per class. Base mana in Cataclysm levels went up a crapload. For example, base mana for a mage at 80 was around 3300. Base mana at 70, around 2200. Base mana at 85? Around 18000. Fireball cost was I think 12% base mana, so when a large amount of your mana came from Int from gear, not base mana, it wasn't a problem. Now that your base mana is up, a fireball that used to take 300-400 mana costs around 2800 mana. This functions the same for healers, all spells got incredibly mana heavy and incredibly inefficient.


edit: That came out as a wall of text. Let's put it this way, Mana costs for spells went up by a lot, and max mana didn't really go up enough to accommodate new costs. Sure, you have 80-100k mana, but your spells cost 2-5% of that per cast.

 

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