This was also my first real test for raiding as a Discipline priest. I really like the healing mechanics, but the tree is still buggy as hell. They have promised to fix all of the bugs by the next patch (or at least declare them to be "features" if they are too hard to fix), but I think the Discipline Tree needs a bit more work if it is going to be viable for anything much more intense than we were doing last night. Discipline allows for some good bursts of emergency healing (Pennance is one of my favourite spells in the game), but by the end of the night my healing output was about 2/3 of the other (Holy spec) priest in the raid. She has similar gear to mine, and based on what I saw in Recount, a very similar healing style. I would (generously) estimate that mitigation mechanics in the Discipline tree might account for about half of that gap, which would make Holy about 25% more effective than Discipline for healing. That's a pretty big gap.
I have some ideas about how I would fix it, but that's a post for another day.
I have also been running more heroic instances lately to try and gear up for Naxx. atara and I tagged along on a quick guild run of Heroic Utgarde Pinnace yesterday morning. When I say "quick", I mean that it turned into a 3-hour marathon grind. A couple of times it was because I dropped the ball (I didn't hit "Fade" at a couple of key points), but for the most part it was because our second warlock on the run is a damage-meter whore, who kept pulling aggro on himself by using AoE on virtually every pull. A couple of guild-mates talked me into a (mostly) guild run of Heroic Halls of Lightning this morning. I was reluctant to go because I had heard horror stories about this instance in heroic, but we breezed through it in about 35 minutes (and the healing was remarkably easy). In retrospect, all healing is going to feel trivial for awhile after yesterday's UP run.