I did Naxx yesterday with some alts in our guild. I haven't gotten any alt to 80 yet (on my mains server) so I "had" to go with my good ol' priest. I did it most for funsies and to gear some guildies since there isn't much for me to get in naxx 10man anymore. I have all the gear and achievements I want by now. I also wanted to give holy and disc another testing area. Remember when I talked about disc being the new black? Although the run to Naxx I mentioned back then got cancelled I still wanted to see if Holy really had become (basically) useless or if Ulduar just offered a new type of experience which demanded a different heal-style.
So when healing Naxx yesterday I noticed one big difference (besides the obvious, Naxx being far easier than Ulduar overall) - there is way less aoe dmg coming in. And when there is, it's usually far apart. I switched between the two specs back and forth during the run (we left Grobbulus, Gluth, KT and Sapph tho) and found that they worked about equally well. On bosses like Loatheb, holy has the clear advantage. You basically only use 1-2 heal spells during the entire encounter, and it's aoe spells. So what is the problem with Ulduar then? Well, even in Naxx I noticed that holy really has less regen. Significantly less. Fights which took me to 80% mana in disc took me to 20% mana in holy (yes we actually wiped on some encounters due to undergeared alts ^^). And even tho they nerfed disc regen, by making the shield slightly costlier I still nearly regain all the mana used on casting it when Rapture procs (shield costs 666 mana and Rapture regens approx 580 mana).
We had three healers with us actually, one disc, one holy and me switching and it made me also notice another thing, or rather get confirmation since I had foreseen the problem already. Having two disc priests in one raid is not such a great idea. Where most healers complete eachother, disc rather "take" eachothers heals. If someone already has shielded my target, it means I can't, and I lose alot of my buffs like the possibility of Rapture procs and Borrowed Time (and others).
So to the conclusion;
Although Ulduar demands more aoe-healing than ever, and that usually is where Holy comes to save the day, the regen nerf to holy makes it basically impossible not to oom too early. The encounters in Ulduar need more regen than Holy currently has, which is sad because healing wise Holy works well for many encounters and really being able to switch between them to use the priests healing possibilities to the max would be awesome. Hopefully they will solve this somehow.