Word is that Blizzard is planning to significantly decrease mana regeneration in patch 3.1. I can see their point: Yesterday I did some farming of level 80 monsters with my priest, and couldn't manage as hard as I tried to deplete my mana. I chain casted smites without end, but being specialized in mana regeneration gear and having put 3 points in spirit tap, my mana simply always stayed nearly full. By the time I done the three steps to attack the next mob, I was at full mana again.
I already wrote about how useless "mana efficient" spells like Greater Heal have become in an environment where people very rarely run out of mana. It used to be that the raid wiped when the healers ran out of mana. Nowadays bosses need an enrage timer, because healers don't run out of mana.
But of course changing the way mana regeneration works will have a huge impact on class balance. In healing suddenly mana efficiency could come back into favor. In damage dealing less mana means a fundamental shift in power between classes that use mana to deal damage, and those who don't. I wonder how all that will work out, and how many more changes to classes Blizzard will have to do in WotLK before they get class balance right.