It seems like the MMO industry is moving away from mana as a resource. Warhammer Online has completely done away with mana, opting for an Energy-like regen system. Warriors and Rogues in WoW use Rage and Energy respectively, and the new Death Knights uses Runes/Runic Power. Even if you look at the mana-using classes, more and more of them are moving away from the tradition version of mana.
The tradition definition of mana is a resource which powers your abilities that starts at full, is slowly depleted, and can only be regenerated out-of-combat or through long periods of inactivity.
If you look at the current WoW mana classes, many of them are moving towards a self-contained cycle for their main resource. For example:
Warlocks: Cast Spells -> Lifetap -> Drain/Heal
Enhance Shamans: Use Abilities -> Shamanistic Rage
Protection Paladins: Use Abilities -> Regain mana through Spiritual Attunement
In WotLK, more classes will join the new model:
Hunters: Full Damage with Aspect of the Hawk -> Mana Regen with Aspect of the Viper
Ret Paladins: (hopefully) Use Abilities -> Judgements of the Wise
The two categories left which use the traditional mana model are mages/boomkins/elemental shamans and healers.
So why are more and more classes shifting away from the traditional mana model? I think the main reason is that mana does not really handle fights of varying length well, especially for DPS classes. If the fight is too long, you run out of mana. If the fight is too short, you have mana left unspent. Indeed, this was part of the reason chaining mana potions was so powerful, as it allowed you to tailor your mana pool to the length of the fight.
As well, in theory the traditional mana model is supposed to make you trade-off longevity for damage. In practice, most people just blasted away with their main nuke, and didn't really adjust tactics in-game to compensate for fight length. The only class which really considered longevity are classes with a 2-cycle spell rotation. And a 2-cycle rotation is perhaps overly sensitive to fight length.
The self-contained model, on the other hand, scales to any arbitrary fight length. The length of the fight is no longer restricted by the mana of the players, it can be as long or as short as desired.
This didn't matter as much in 5-mans, as 5-man fights have an implicit timer: the healer's mana bar. When the healer's mana bar runs out, the fight usually ends. As well, unlike DPS which has very little "over-damage", healers can extend their mana bar by playing smarter and avoiding overheal. Wasted mana is very important for a healer, unlike DPS. However, once you have more than one healer, a healer's mana bar can last a lot longer through efficiencies and trading off regen time, or even just reducing the damage done. Unlike DPS, the rate at which healers spend mana is very dependent on the specifics of the encounter.
I think time has shown that the traditional mana model is not the best resource for an MMO. In a lot of ways, it is either too constraining, or ends up not mattering at all. I think that more and more MMOs will move towards self-contained resource cycles to power abilities.