1) You care more about mana regen on gear instead of just throughput. This is the least important consideration because it still works today to some extent.
2) You care more about your whole arsenal of heals instead of just the biggest, fastest one. Paladins for example currently can go for a Flash of Light build or a Holy Light build but they aren't often mixing the two together. A Holy Light focused paladin isn't using FoL to hit the tank from a small hit. They're just using HL and overhealing.
3) Boss encounters don't need to be built around destroying the tank in 2 hits. Currently, that is one of the few ways we have to make players actually lose a fight. (The other is that you lose enough healers or dps through attrition that you just can't keep going). Healers running out of mana used to be another point of failure and it pushed things in slightly different directions. Tanks for example cared slightly more about avoidance stats because nobody wanted to be the dreaded mana sponge. Standing in fires didn't have to be about dying in 1 sec if you didn't move -- it was more that you wasted healer mana by taking unnecessary damage. DPS who didn't do enough dps risked the fight going long and the healers running out of mana. Currently the long fight is only an issue if it opens up more chances for tank death or there is a severe berserk timer.
[...] We're not sure the five second rule is a rousing success. It's not necessarily fun for players to stand there and do nothing. It definitely wasn't fun to heal in rotation and do nothing for a minute or two so you could regen.
What I'm talking about is just returning HPM (healing per mana) to being a consideration that actually matters again. Do you want to use the cheap spell or the expensive spell? Currently you only really care about the fast spell or maybe the big spell.
Healers will need some tools to manage their mana, and they already have some like Innervate and the Shadow Fiend. Those tools can't be counted on for unlimited mana of course or we're back to the same problem.
...The more careful selection of spells is actually what we're going for. I think that can be just as challenging, and personally probably more fun, than the miss a GCD and the tank gets it style.
But really I was trying to offer reassurance to those healers who seem terrified that we're going to make healing too hard in Cataclysm. That's not the goal. Making it fun is the goal. However, when healing is too easy, it's hard for it to be very fun. As I've said before (maybe in this thread?) DPS specs can try to see how fast they can make a farm run. The healer doesn't really get to try and "heal faster" or whatever. The most fun a healer can have in content on farm status is trying to keep everyone alive as efficiently as possible. Efficiency implies a mana component.