Couple of points...
1) I was trying to argue (perhaps too politely) that the notion that healing was fun when smart people could take healing breaks to allow for out of five-second-rule regen was a little bit of rosy nostalgia. The way expert raiders handled that was by healer rotations or stopcasting macros, and I don't seriously think a lot of people long for the return of those. (No doubt some do.) Sunwell and similar content were tuned to where you couldn't realistically stop casting for regen anyway. If you are stopping now, it's because Naxxramas is easy. On challenging content, all 4 classes already spammed heals pretty consistently. That meant the out of FSR breaks came mostly from clearcasting procs or encounter mechanics that forced you to pause. When that happened, mana felt limitless. As I've said, we would rather shift back a little (A LITTLE) from GCD-constrained healing to mana-constrained healing.
2) It sounds like some of you are saying that even on challenging content without replenishment and without leaving the FSR your mana is infinite. I am a slightly skeptical of such claims, but we'll see what happens in Ulduar. We think you will run out of mana sometimes, particular on very hard fights or if you aren't playing at your best, even with replenishment. Similarly, we feel like the paladins are noticing the Divine Plea change in PvE. If you aren't, then perhaps you are an amazing healer, or perhaps Naxx isn't challenging you much. Now, if it turns out regen is still just too good, then replenishment would definitely be our next target. But we don't think we're far off. This isn't a "to the ground" scenario.
3) This change won't change the relative values of Int and Spirit. Int was already a good regen stat (in part because it provides other bonuses). However, you are rarely ever choosing between a +100 Int chest and a +100 Spirit chest (though you might be with gems). More to the point though, the regen formula is SQRT of Int * Spirit * a constant. We are just lowering the constant. If Spirit was good for you before, it will still be good for you. If the only reason it was good for you before was because you took a lot of out of FSR breaks, well 1) again I am skeptical, and 2) yes, under that specific situation you will notice a nerf to your regen.
Overall, our goal is not to make healing more difficult across the board (though we do want mana to matter more). Our goal is to make healing more interesting.