#1 requirement: the thinnest possible side pieces so as to not allow noise to get under the headset, and to avoid them being pressed into one's head by the headset.
#2 requirement:
not polarised, otherwise anything with an LCD display will be variously hard to read according to whether you sit the right way up and hang upside down
#3 requirement: avoid fancy tint colours. This is debatable according to preference. I have found that neutral (grey) tints make it hard to see buildings/airfields among a lot of green. Brown tint is good for this. Green tint is reportedly the best but makes colours look weird.
The most fashionable glasses, e.g. Oakleys, are generally crap for flying.