And of course your own gameplay preferences, with AoE, HoTs, Mobility, Tank heals, PvP, CC, Party buffs, etc..... Heck, you can even consider their Dps too when there is little to heal in downtimes.
I myself love the angelic AoE theme with Holy Priests, which is why I rolled it. I was so sad when they took Archangel away from Holy and Shadow in MoP :( I also must confess to loving the troll abilities Void Shift and Leap of Faith. On the rare negative, I dislike the themes of Chakra States, which I feel fits Disc way better, but I do love the gameplay adaptions it gives (Red makes doing dailies and other solo content possible).
On equal measure, I've come to learn and love Resto Druid's theme of flora growth. I also love their versatility, single target HoTs and mobility that is abundantly necessary in raids. Symbosis is the ultimate co-op spell too, which is awesome.
This can count into race decisions too, like human or dwarf paladins being badass in holding off the Scourge with the Argent Crusade, tauren Sunwalkers starting to worship the sun, trolls being witch doctors, female nelfs within priesthood for Elune, belf blood knights, male nelf resto druids worship of Nordrassil, gnome doctors, Earthen Ring resto shamans mission to heal the land from the cataclysm, gilnean druids growing harvests before the worgen curse, etc.....
I myself also loved the contrast with worgen being vicious-looking, yet healing others instead, so yeah, that's why I rolled one. Very unique I think.
Although I do love all healing classes as a standard compared to other roles, if I had to rank my inclinations it would be:
Holy Priest=Resto Druid>Resto Shaman=Disc Priest>Holy Paladin=Mist Monk
Anyone else care to share?