According to the official Diablo Twitter account, the team is considering removing mana from Diablo 3- that's the blue stuff - and replacing it with class specific resources instead. While the team had asked for feedback on it, it appears that mana has already been cut from the game for some of the announced classes.
If you listened to Blizzcast Episode 8 you'd know the Barbarian doesn't use mana, but instead uses a new resource called Fury Orbs. To Blizzard, it simply didn't make sense that a physical natured character would drive his power from mana. Makes sense. But in the interview with Jay Wilson, Diablo 3's designer, it appears that even the Wizard might not use mana.
Jay Wilson: Well for the Wizard we want to enforce the fact that she’s a glass cannon. I don’t think it’s fun to ever run out of mana. I’m not really interested in an extended resource for her.
For the wizard, when she’s out of mana she just dies. And that’s not fun. So if anything, we want to encourage how she plays. So she’s the kind of character that blasts first and asks questions later. Very vulnerable. So we want to implement a system that makes her more blasty, but even more vulnerable. We want to make that a choice for the player. “Do I want to make myself more vulnerable in exchange for being more blasty.” And that’s a cool gameplay pull there.
Seems like an interesting trade off, you're more vulnerable when casting, but you can cast all you like. According to him the Witch Doctor will probably still use mana. The Monk - well he's not quite sure, or rather, can't say.
So where does this all leave us? Basically the game is still well over a year away, we're still a character or two shy of a full roster. And there's some pretty major design decisions to be had. But, the lack of a standard resource between characters does effect co-op play quite a bit. No longer will you have to exchange mana potions to get through tougher parts of the game, granted, it appears Blizzard may have mostly dropped the potion concept in favor of orbs - much like it did with the game's health system.
Do you think having class specific resources adds another layer of depth to the game or another layer of confusion?