I don't know what your thoughts are, but I found myself thinking about what spec would be optimal for each new fight in DS yesterday. We got up to Spine of DW and most of the time I ran as A/AA Disc, which is nowadays my "default" for starting out. I heal alongside a hpaladin and a resto druid in ten man.
However, it felt like on quite a few fights, Holy would be a really good choice, although how does it hold up on the regen front? The AoE nuke healing fights like YorSahj and Zonozz seem really good for Holy, although Disc was simple enough to handle there and with 3 healers in ten man, the non absorb healers will do a lot of overhealing. Hagara I can't even remember much of, except for thinking that Body and Soul could be nice and apparently DH heals the targets inside the ice tombs.
On Ultraxion, I ended up taking Blue as Disc, with paladin on green and druid on red. It worked well enough, but are there other/better combinations? Smiting to keep Evangelism up towards the end can be a bit tricky as A/AA when things are going south with people's health.
In the end I went SoS Disc for Warmaster as there just wasn't much time to smite and a lot of movement made it hard to micro manage evangelism stacks. Found myself thinking Holy would quite good there as well with increased mobility for soaking and Lightwell for more healing on the move. On the other hand SoS Disc was neat for the tank healing in P2.
Not sure I have any thoughts of spec on Spine yet, although it seems anything will do. Finding a good target to smite in the mess adds another element on confusion in the beginning.
EDIT: Not trying to create a "spec war" or anything, just interested to see what people's thoughts were so far and what sort of tips and tricks were out there.