I prefer a less variable mechanic for limiting fight length - enrages or stacking debuffs.

There's a very simple reason for this: if you base the fight length solely on mana, then the fight can run indefinitely. Just stack more healers. If healer mana is the limiting factor, then once you reach the point where healer regen is equal to healer expenditure (via incoming damage), the fight IS endless. Essentially, using mana to limit fights gives them the potential to be limitless.

Consider Heigan, in Naxxramas. Many of us remember raids that were decimated by the Safety Dance, only to watch a duo of healer and tank remain endlessly whittling his health down. I remember several times where the fight ran for 15+ minutes, with the dead bodies clamoring for us to "just die" so they could get back up and do something. With mana being the only deadline, once the healer was only healing two people, the damage was easily manageable. Fun, right?

Other factors can be more easily controlled. Stacking debuffs or hard enrage timers set a specific deadline. Doesn't matter how well you can deal with incoming damage; if you can't kill the boss before the deadline, you're done.

I'm 100% for replacing mana with active models. At that point, the healers aren't playing a double game by also defining the limit (or endlessness) of the fight. The whole point is whether you're strong enough to heal the damage.

Whoops, I forgot. People went into the healing game because they liked being fancy egg timers.

My bad.