As a blanket statement, healer mana wasn’t a big concern in Wrath of the Lich King. You could run out of mana sometimes, but it really didn’t affect your spell choice in the way it did prior to Lich King. We think resources should be important, though. A lot of gameplay in a wide variety of games comes down to managing a limited resource, whether it's Vespene Gas in an RTS, ammo in an FPS, or even time in a puzzle game. Managing your resources well makes you a better player. Not being limited by resources can feel empowering over a short period of time, but only because it feels like you’re breaking the rules. In fact you are breaking the rules, and once those short periods of time have ended, a game can quickly lose its luster. Godmode isn’t nearly as compelling in the long term as it might seem at first glance.
Now, it is true that resource management is an even bigger part of the game for healers than it is for other roles. “Not fair!” you might be ready to cry. I used this analogy once before, and it seemed to resonate with lots of people, so I’ll use it again. Dealing damage is like a sprint. You typically want to go as fast as you can. Healing isn’t a race though -- it’s more like darts. You want to be as precise as you can. A big part of the healing gameplay is using the right tool for the right job. The resource cost of those tools is one of the things that differentiates them. Remove the resource constraint and you lose one dimension that differentiates the tools. Good healers used to pride themselves on keeping everyone standing up without running out of mana.
For a number of reasons, all of which were completely our fault, healers had too much mana regeneration in Wrath of the Lich King. Let’s look at the consequences of infinite mana for a moment.
For starters, those expensive, fast heals were never a difficult choice. Expensive doesn’t really apply in the absence of a cost, so they were just fast heals. Why wouldn’t you want to cast a fast heal? Healer gameplay became smaller because they had fewer options. Rather than choosing the right tool, everyone picked a spell such as Power Word: Shield, Flash of Light or Rejuv, and just used that spell. Over and over. We think a cornerstone of good gameplay is making interesting decisions. When your toolbox is too small (because the expensive or slow spells are immediately discarded as tools) then you are making fewer interesting decisions.
Second, since healers weren’t really running out of mana, we had to find other ways to make those raid encounters that were designed to be challenging actually challenging. That often came down to very high tank or raid damage. So now not only did healers not have much of a choice about which spell to use, but they also had to use that spell every global cooldown or risk someone dying. This made healing stressful without the reward of having made good decisions. If you healed the wrong target, hesitated for a moment, or had a laggy connection, then someone was going to die.
Third, anything that played off of mana regeneration, such as a talent, a stat like Spirit, or even a proc from a trinket, became undesirable. Furthermore, since mana wasn’t a concern, overhealing was also not a concern, and players did it with abandon. When everything is an overheal already, then stats like critical strike chance also become devalued.
Fourth, PvP balance suffered. When healers could easily heal anyone to full without fear of overhealing or running out of mana, then battles became very binary. You either killed someone or you didn’t. Nobody sat in a wounded state very long. There was no sense of a changing tide or someone coming from behind. Imagine a tennis match where the outcome of the first serve won or lost the entire match. We could have improved this situation by increasing health pools, which is exactly what we did for Cataclysm, but larger health pools with infinite mana would just make bosses feel unthreatening.
To be clear, we don’t want healers to constantly run out of mana. We want them to run out of mana when they don’t play well. And we don’t want them to always fail. But we do want them to feel good when they are challenged, and overcome those challenges to succeed. When someone is wounded, we want healers to consider whether to use a slow, efficient heal (because they aren’t in immediate threat of dying) or a fast, expensive heal (because they are). That’s called triage, and it was notably missing from the Lich King healing environment. We think triage will make healing more fun. We’re making this change not to make healers sad by nerfing them, but to make healers happy by making the game more fun for them.