They've just tried really hard to make healing less fun this expansion, it seems. Boring talents and artifacts. Unrewarding masteries. Even complete alterations of specs like disc, giving them a higher skillcap (and higher frustration-level) than any other spec for no real reward.
Doesn't help that the healers don't have fun off-specs.
They should've given DHs a healing off-spec. Called it HATE. That would be its resource, too. Heal people with your hatred of them. It's regenerated passively, but also fueled by them taking avoidable damage. And make 'em a fucking melee healer, too; who gives a fuck? Means you don't have to rape holy paladins more than you already have just to shoehorn something like that in there for whatever reason. Shit, the thematic and class-fantasy is so fucking perfect. Real lost opportunity there, blizz.