The discussion about Illumination has me thinking about mana regeneration in WoW. Paladins have spell crit, Warlocks have Lifetap, but what do priests/mages/druids have? They have Spirit.
Part of the reason that there is so much conflict over regen and endurance is that, quite frankly, Spirit is a terrible stat and mechanic. Spirit is widely considered the worst stat in the game, and gear with Spirit is generally regarded as much less valuable than gear without it.
The problem with Spirit is that it violates what I will call Coriel's First Law of Design:
Doing stuff is fun.
Not doing stuff is not fun.
Mana regeneration via Spirit is governed by something called the Five Second Rule (FSR). The FSR states that you only get mana back after you have gone five seconds without casting a spell, and you only get mana until you cast another spell.
So not only do you have to do nothing to regenerate mana, you have to spend 5s doing nothing with no benefit before you even start regenerating mana. And 5s can be an eternity, especially for a healer.
Contrast this with Illumination or Lifetap. Illumination rewards the paladin for casting heals. With Lifetap, the warlock is actively controlling her mana regen. In both cases the player is actively doing something worthwhile to improve their play. They are not standing around doing nothing.
Spirit is so bad that we actually have a second stat which governs mana regeneration: mana/5s or mp5. Both stats return mana to the player, they just differ in how they do so. Mp5 returns mana at a continous rate, even if the player is casting a spell. Mp5 is widely regarded as superior to Spirit because it works all the time. A player with mp5 can actively play the game, and not just wait around.
The fact that we have two stats which basically do the same thing is a really bad sign. It's a clear indication that the original stat mechanic was deeply and fundamentally flawed.
It's probably really late in the game's lifecycle to change this, but I think that Spirit should do the same thing as mp5. Return mana to the caster at a continuous rate during combat, and return heath/mana at a higher rate outside of combat. All items with mp5 should be converted to Spirit, and all talents that allow mana regeneration while casting should be reworked to provide bonus regeneration.
At one fell swoop we eliminate a stat, simplifying gear choices, and make Spirit a valued stat once again. As well, priests, druids, and mages get a method of mana regen which can be counted upon and which lets them be active players having fun, while still getting the benefit.