Mana is one of WoW's biggest sacred cows. Paladins, shaman, druids, mages, warlocks, priests all make use of it, and hunters were once also on the mana teat, as it were. Every healer uses it, and when the monk class is introduced, they'll heal with mana as well. Every ranged caster uses it. It's the resource system the majority of WoW players are most familiar with, a pool that starts at full and empties as you use it. Over the course of its existence, stats like spirit and MP5 have filled it back up during combat, keeping those classes that rely on it supplied. Since it's the lifeblood for all healers, it effectively is the same for all tanks, even though only one tanking class actually tanks with it. Two classes use it to melee DPS, both hybrids, and these two classes effectively ignore the regeneration of the mana pool via talents and class abilities that make mana regeneration a non-issue.
Mana is fairly easy to understand. You have it, you use it. There are various systems built in to make regenerating it easier. With the addition of runes and runic power for death knights and holy power for paladins, secondary resource systems (similar to the combo point system of rogues) have also been introduced to the game. Holy p-ower in particular is interesting to this discussion because it is a secondary resource added to a mana class and one that works for healing, tanking and melee DPS. (Everyone has their own opinion of how well it does so.)
This leads us to the subject of this post. Do we need mana at all? As my intrepid coworker Michael Gray pointed out to me when discussing this article, mana serves many uses. It's not just that it's a resource for healing and DPSing, but the finite nature of the mana system serves to limit encounters in both PvP and PvE. Doing away with mana could have as many negative effects as positive ones.