Bass quantity/quality, midrange quantity/quality, etc. Just emphasis on the frequency range (or lack thereof) that I might happen to prefer. However you would want the sound to be presented. Technical performance like soundstaging (how large the uhm...stage size is), imaging/positioning (how accurately they position each aspect of the sound relative to the recording), transient speeds (how they keep up with note pacing), note presentation/decay (how long the notes last for), etc.
As an example, would personally would go for something linear, tight impact, fast, extreme clarity and transparent, smooth but acceptably emphasized treble.
Closed-back or open-backed is another issue. Would also be useful to point out how you'd use them (at home, on-the-go).
Anyways, the CALs! Mice mentioned would be a fine starting block point (wtf's a block).
Edited by TwinQY - 3/24/13 at 12:04am