Just some thoughts:
Be good if the double can be converted to a single, just gives you more flexibilty.
Nice if its easy to tip on its side for storage.
Built in loading ramp or facility to carry a ramp on the trailer.
Able to load bike single handed.
Jocky wheel so you can move the fully loaded trailer around - some people find reversing almost impossible so they un hitch and push the trailer into position.