(1) Soundstage/headstage... probably the most important aspect of headphones to me. This is why I'll never like Grados, period.
(2) Dynamic range... has to be capable of reproducing music with a wide dynamic range well. Macrodynamics.
(3) Air/Delicacy... capturing the most subtle nuances of the music, and with an "airy" quality (ala Senn HD600).
(4) Neutrality/naturalness. Has to be both. "Clinical" sounding neutrality doesn't interest me, how are you supposed to enjoy the music?
(5) "Softer" treble, warmth, whatever you call a quality that makes it easy to listen hour after hour without ear fatigue. Probably several qualities fall into this category.
Detail has practically dropped off my list lately, although I like it from an "audiophile" perspective. Often, really detailed/revealing gear has me listening either to the gear itself or the recording quality. Very detailed gear often makes recordings sound like recordings and IMO kills the illusion with live stuff. Studio-recorded music, it doesn't matter so much but I have to be able to listen the music without being distracted by the gear. I think Rick hit that one dead on.
P.S. my list of priorities is constantly in flux, and changing. That's not to say I can't be perfectly happy sticking with particular gear for a long time. As long as it has no downright offensive qualities, I can enjoy it.