Screwface on the forums has an interesting idea about a PvP tweak, even though his implementation isn't quite right. He says that since healers are so overpowered in PvP (well that's his first problem), abilities like Rend and Garrote should not only bleed blood, but also bleed mana off of casters. Of course, simply making all bleed abilities also take off mana would make them overpowered on their own, so his plan of making a sweeping change like that doesn't quite compute.
But the idea of more abilities that directly affect mana is an interesting one. Right now, there are only four "mana drain" spells in the game (warlocks can steal mana for themselves, priests can turn mana into damage, and hunters can sting mana off of a target). But as much as mana pools and regen have grown in the last patch, it's true that there hasn't been a balance in the opposite direction. No, warriors don't need another buff, but what if shaman were given a mana drain totem somewhere in the next ten levels? Or Boomkins got a spell that negated mana over time?
It's nothing to play around with lightly. But Blizzard does have to come up with ten more levels of abilities and talents for the next expansion, and messing with mana is something they haven't done much of lately. In Northrend we might not only be worried about health and DPS, but mana draining and mana attacks might become another piece of the class balance puzzle.