Let's just get this disclaimer upfront: This is not intended to be based on class changes reminiscent of a FoTM decision, although the "look" of where a class "might" be going can pique your interest. Obviously Alpha changes are not final, and basing decisions on them currently would be silly.
Personally, I only started playing very late cata (reaching 85 just as the MoP pre-expansion patch hit) and have enjoyed all three priest specs throughout the expansion, though I quickly discovered that my skills were much more suited to healing than to DPS, and over the course of the expac I've levelled one of each healer to enjoy and study. I simply love healing, and learning alternative styles, specs and methods. My main, is a priest, however. I do try to play all of my healer alts, but it can get a bit tedious, so I've sort of been Priest (main), Resto Shaman (main alt) throughout.
I've always really enjoyed Holy spec, though at 85 played mainly disc before MoP hit, so I continued to do so. At the release, disc was absolutely horrific, so I attempted to play holy but didn't quite understand what was going on with it so left it aside for a while. Upon learning holy properly, I found I absolutely loved the spec, but generally, I didn't have much opportunity to play it until late ToT and into SoO on farm (or if there was another disc priest). So for WoD, I am looking forward to dusting off my Holy spec and perhaps giving it my attention as a "go-to" as I feel given the power disparity, I've not had enough opportunity to experiment with the spec as my most familiar thus far. Of course I'll still run Holy/Disc, but I'd like to give holy a bit more attention!
In terms of alt, I'd quite like to give my druid some love in WoD, since it was the last healer I got to 90, and I never got too far with it, bar a few casual normal siege kills. I really enjoy the style, and the utility, and I'm quite excited to delve deeper into its nuances.
The thing is most healers also have at least one other healing alt. What healers and priest specs are you interested in for WoD, and why?