Mana, as a resource, is important only to healers and Arcane Mages. It is, at best, a little inconvenient hiccup to deal with as an Affliction or Demonology Warlock, but for Destruction, Shadow, Fire, Frost (Mages,) Balance, Elemental, Enhancement, Retribution, and Protection (Paladins,) it's a pretty blue bar that sits at full at essentially all times.
Now I like that blue bar. Blue's my favorite color. But that's about the only reason I like it.
To be fair, most of these classes have ways to waste the resource. Mages can spam spell-steal to remove buffs from enemies and take them for themselves, and most of the listed specs have some healing spells that would normally be used a lot more by their healing specs, and these will drain the mana to prevent those classes from just spamming heals and supplementing the real healers.
But for a more modern example, Monks typically use Energy, and they use that to cast their healing spells, which puts a pretty strong limitation on their ability to supplement the healers.
Building a resource system isn't easy, if you want it to feel unique. Monks went with Energy because a quick-recharging resource like that would pretty much just feel like Energy even with a different name. I actually wonder if, should they have to do it all again, the Hunter Cataclysm revamp that changed their resource from Mana to Focus would have just used Energy instead.Focus is of course inherently more limiting than Energy (having a passive regeneration rate at about half of what Energy gets) and so they built Hunters around this idea of spend-and-build.
A lot of the elements of WoW's combat gameplay came out of the Diablo series. Diablo III's combat system actually gives a different resource system for every class (and of course, the way abilities work, it's possible for two players of the same class to use an entirely different set of abilities.)
All the resources are different, but there do tend to be common themes. The Witch Doctor's Mana and the Wizard's Arcane Power function the same way, except that the Wizard burns through his or her Arcane Power much more quickly, but then regenerates it very quickly - allowing them to unleash their most powerful spells, spend a few seconds casting their free "Signature Spells" and then going back to their big ones. Witch Doctors can front load a lot of their very powerful spells, but they need to spend a lot of time (relatively speaking) easing off to allow the Mana to regenerate (more of their spells have cooldowns - something I'll get to when talking about Paladins and Shamans.) Demon Hunters' Discipline also functions somewhat like this, but you have your Hatred-related abilities to keep yourself busy if you need to wait for Discipline to regenerate.
Demon Hunters' Hatred, Crusaders' Wrath, Monks' Spirit, and Barbarians' Fury all function as build-and-spend resources - though Fury is the only one that sits at zero by default (it's very similar to a Warrior's Rage, but there are no auto-attacks in Diablo III, so generating Rage is an active process.) These resources are made to feel pretty different, though, by varying how quickly they are built up and spent. Hatred and Fury both move pretty fluidly in both directions (and they have some design space to feel similar, given Fury's default state and Hatred's coupling with Discipline.) Wrath gets spent quicker than it is generated, and Spirit probably takes the longest to build-up/recover before getting unleashed in only one or two attacks.
The only real rule here is that spenders have to be more powerful than generators, as otherwise players would just spam the generators.
Frankly, I think that with a lot of secondary resources that currently exist in-game, you could almost just get rid of the mana bar. Shadow, Destruction. and Demonology kind of have the rhythms of their specs defined by their secondary resources. You could kind of say this about all the non-healing Mana specs, though something like the Balance Meter isn't exactly a "resource" per se. And of course Elemental doesn't have a secondary stat unless you count the Lightning Shield charges.
Melee specs that use Mana rely on giving almost every attack a cooldown. It would then be tempting to give Paladins or Shamans some kind of Energy-like resource instead. The only problem here is that you wind up losing some of the motivation to use multiple attacks. If I could spam Crusader Strike instead of weaving in Judgment, I'd be able to do more damage. So those cooldowns serve a purpose. Rogues and Feral Druids get around this by making the generation of combo points less interesting than the weaving of finishing moves (though even they have multiple generators that should be used in each spec.)
I could imagine a version of Ret and Prot Paladins who use a resource called Righteousness - perhaps having to use Judgement to build it up and then spend it on things like Crusader Strike and Consecration. Maybe Hammer of Wrath would cost prohibitively too much Righteousness unless it was used below a certain threshold or during Avenging Wrath.
This would pretty much eliminate spare global cooldowns within the rotations, as you'd be hitting Judgment any time you didn't have enough Righteousness to generate the Holy Power.
But the danger here is a kind of "don't fix what ain't broke" problem. Right now, even if Mana is kind of superfluous to these specs, it might not be a such a great idea to mess with what exists. Sure, Destruction would be essentially unchanged if you just got rid of Mana, but creating a new system just because Mana is redundant could unbalance some perfectly healthy classes.
So I advise caution.
Still, I'm really curious to see what Insanity will be.