The idea of lvling a healer is get a second spec that may share gear with healing and try to learn both. Druids and Shamans can do that. Priests could but you may also want Holy/Disc :)
Shaman can do that really ok going elemental. Like druids prepare for 5 min fights each mob if you aggro in a healing spec ...
But with the newest boomkin nerf it will take even more damage so in the 85-90 bracket i believe it will be even worse to lvl than it already is. So Resto is one of the worse lvling spec in the game and add to that that boomkins arent that good and druid will have a tough time.
Priests arent bad with Disc and Holy is passable. SPriests afaik (and that is not that far) might be ok even with their nerf.
Paladins .... Those are in trouble ... Holy is not only terrible to lvl but will be worse. No mana back seal, judgements more expensive and full holy powers all time with no spells to dump it. I believe that Holy paladins will really need an attack spell to dump holy powers and ease out mana consumption.
Monks will need to bring another set (like druids ...). At this point i think that Monks really should be agility based and use energy even for healing. Mistweavers seems ... strange.