The original comment may not make sense the way it is worded, but its been my experience that priests struggle mightily in 5-mans (including heroics) as well.

I tank, nothing more complicated than 10-man VoA, but at least 2 heroics a night, and my shoulders slump when I see the healer is a priest. I have never rolled a healing spec, so I can't really comment on the mechanics of it or what is happening or why, but druids, shaman and especially paladins don't have any trouble at all, even in AoE-healing-intensive dungeons like Azjol-Nerub or the Oculus.

I've run with top-notch Holy paladins who know how and when to use Bacon and can group heal like nobody's business.

Seeing as they changed things to move in the direction of player > class... in before what people will call flamebait... maybe there are just a lot of lousy priests who were exposed to the world after the lolpatch and Wrath's release.

Not saying this is the case, I'm saying its a possibility. As I said before, I would be like "How do I shot heals?" as I've never spec'd for healing before, but I'm just raising a possibility.