It's all subjective. Everyone's definition of "quality" is different and everyone's interpretation of sound is too. As far as what to "look out for", it all depends on the situation. For example, I was looking for a pair of cans for use with my Zune a while back; the Zune performs best when used with headphones with an impedance of 32ohms or less so the cans I tried on it that were 64ohms, while being high quality cans, just didn't sound good on it cause I had to crank the volume to max just to get it to play at an acceptable level, and even still it was barely acceptable.
So the main thing is to familiarize yourself with the equipment on which you'll be using them mainly cause while the 64ohm cans I tried would sound heavenly on my home theater receiver, they just wouldn't cut the mustard on my Zune.