1. Isolation is preferred (doesn't have to be completely isolated though, just enough to keep out some noise). One reason I'm getting portables is because my regular portable headphones just don't keep enough sound out.
2. Comfort is a big one. I sometimes have problems with in ear headphones due to cable noise and a pressure feeling in my ear that I just can't get used to. Some in ears work and others do not in my experience.
3. Portability. I'm going to be bringing these to and from classes, work, home, etc.
4. Durability. I need these to be of decent build and not snap in a backpack.
Headphones I have looked at so far....
Sennsheiser HD-25 II -- Above my budget so I really prefer not to get these.
Audio-Technica ATH-EW9 -- Looks pretty great. I'm a little worried about isolation though.
Audio-Technica ATH-ES7 -- Seems decent, but not very portable.
HARMAN K81DJ Akg
beyerdynamic DT 235
I'm definitely not limited to these choices. Reccomend me some others too!
EDIT: Forgot to say that DJ headphones are great too. There are just so many of them... I have no idea where to start looking. AKG seems to have good ones, and the Koss - Pro DJ100 has had good reviews. Have never head ultrasone so I'm not too sure about that brand.