To see how this has played out in Cataclysm, consider druid healers as the expansion has progressed. Beginning healers use nourish almost exclusively, considering healing touch ridiculously expensive. Firelands raiders, meanwhile, use healing touch almost exclusively, and have little reason to bother with nourish. Additionally, I can attest that in quest gear, the level 85 heroics are nearly impossible to heal without running out of mana. Yet, again, in Firelands, mana just isn't a concern. You can't be stupid about it, but I no longer feel like I have to wait to refresh lifebloom, or that I need to switch to nourish whenever possible, or that I should bother cancelling a heal if I see someone else landed a heal ahead of mine. I just take it easy and worry about other things than mana.
There are a number of ways to fix this situation. One of them is to stop giving more mana with more gear, but I think that would take away a lot of the character of a mana-based class. Assuming they don't make that change, the only alternative is to make the cost of spells continue to increase at higher gear levels.
There are a number of ways to make spells cost more. Here is a simple one: Intellect increases both the power and the cost of spells. Intuitively, the idea is that intellect allows healers to use more mana at once. It's like using both barrels of a double-barreled shotgun: it does twice the damage and uses twice the ammo. From a balance perspective, as healers gear up, they would observe not just their regen improving, but also their rate of resource usage. They'd have to balance these increases with their gear choices.
Blizzard has already decided that in Panderia, intellect will no longer increase the size of the mana pool. That change matches well with the one I describe. I wonder if they will take a try at it? Throughput versus regen is one of the more fun itemization choices in Warcraft, and right now you only have much of a choice at certain particular gear levels.