1. Bang for the buck
For me, being 70 years old and with declining hearing:
isolation- allows me to block extraneous noise and concentrate on detail
clarity- that which makes a clarinet sound like a clarinet, and a flute like a flute
realism- that wow factor that I'm really there and that the players are in the room with me.
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.
My favorite features of an IEM...
1. Detail - capturing instrument separation and subtle tones around vocals...
2. Presentation - balanced, open, grand imaging that sounds natural and real.
3. Comfort - Designed with the listener in mind. Fit, cables, ergonomics, carrying case all should support the IEM and it's daily use.