The answer would be me (and no, I'm not talking about arena; that's a whole different animal). What do I mean, then, if not PvP? I'm talking about the difficulty in changing from a healing perspective to a DPS perspective
Besides one or two isolated cases, I have never done serious DPS. I've done a few five mans on Bellbell, one Karazhan, a few Outlands dungeons on Sugarcake, and one stint in Karazhan as a Boomkek. Since I began actually understanding how the game was played, I've been healing. I have learned through what annoys me or my tank some of what DPS does, but more what I should not do. That's why I'm mortified if Cinnamon has his Growl on, or if I hit the wrong mob, pull aggro and die.
I've spent two years working on my healing. I'm far from perfect, but I'm always striving for it. So placing myself in a DPS position is a practice in suppressing my urges to drop out and heal (especially when I'm playing Bellbell and I don't believe the healer is doing a sufficient job; it's easier to resist when playing Sugar since she can't heal anyone but her pet). Unless there are marks, and there often aren't, I haven't learned how to prioritize targets to kill. Obvious exceptions would be bosses; I've spent too many times explaining why you should kill the adds on this boss and not on this boss to not understand those.
Organized DPS is also a supremely unforgiving environment, at least in the PUG world I currently reside in. Not due to the game itself; without CC being a requirement or necessary except in the most extreme cases, DPS is free to go all out with relative abandon, its only focus being "kill that big spider thing, and maybe get somebody webbed off the wall." DPS is most unforgiving due to meters; the highest placed person is touted, the lowest place is ridiculed. This makes a highly hostile environment for the new 80, the undergeared, and the inexperienced (or newly respec'd). I've seen, and tried to prevent, people being ridiculed or their opinions being invalidated due to a couple hundred points in damage difference.
Besides, everyone seems to expect you to just know what to do. Perhaps not on boss fights, but they expect you to understand how to use a DPS rotation, how to spec, how to gear, how to gem, and just what to do. And though I have many of the concepts understood, putting them into practice is an entirely different animal. I've improved with Bellbell, I believe, but Sugarcake is Beast Mastery, and managing both melee and ranged components (competently) is almost overwhelming. I barely avoided breaking two CC's by quick switching Cinnamon's targets, while I accidentally pull the mob with the least threat on the tank and forget that Oh yeah, I can feign death.
With Bellbell just reaching level 77, I have a lot of catching up to do, and only a little time to do it.