Mana regeneration has been a hot issue for raiding healers during this expansion. Basically, we had far too much of it in tier 7 content, to the point where we could ignore our mana bars in many situations. I once joked that I could replace our holy paladin's mana bar with a blue piece of paper without any loss of information; it wasn't very far from the truth.
When tier 8 came around they tried to nerf our mana regeneration so that we'd finally care about overhealing and gear for mana regen as well as throughput, but it seems they didn't go far enough, at least for one class. Many Holy Paladins are still ignoring MP5 gear; in fact, the aforementioned holy pally refused to pick up most Ulduar spell plate because it all has MP5 on it and he likes his Naxx-25 stuff better.
He's not alone in this tactic; many paladins are taking the same route, and mourning that this gear is no good for us. Ghostcrawler recognizes this problem, stating that "Regardless of any other changes we might make, mp5 just doesn't seem to provide enough point for point on gear." So what are they going to do about it? He says "It's possible (likely?) we'll just increase all of the mp5 on gear."
I'm honestly not sure this will solve the problem, since I suspect lots of people still won't want MP5, but it's a step in the right direction; MP5 has historically been a very expensive stat, which means the other stats on a piece of gear have to be lower just to get a little bit of mana regen out of it. And he does mention "other changes we might make," so I guess we'll just have to wait and see.