Why is the healer class basically a one spell dps class?

[Lyth]

...

There's tends to be a fairly narrow perception of what 'buffs' and 'utility' entail. Buffs are really only interesting when there's some element of prioritization to them. Systematically clicking on players and applying 'damage up' is about as interesting as throwing rocks at the boss.

I think that buffs are most interesting when you're dealing with mutually exclusive ones which cannot directly be compared in terms of effectiveness (i.e. movement speed vs. regen). This works especially well with an aura or totem system, but none of the present healers have anything resembling this. It doesn't have to be particularly complex or involve lots of choices, as long as you're constantly required to re-evaluate what you have up.