1) Comfort - It is pointless to have great sounding headphones if they are uncomfortable to wear
2) Construction - this covers durability, ease of use, folding/portability, quality of cable, etc - once again, great sound is pointless if it is too delicate to handle and/or too inconvenient to use
3) sound reproduction - for casual listening I'm actually not too picky about the actual sound quality, but there should at least be adequate bass response, limited treble peaks, and sufficiently quick decay times that sound isn't blurry.