The windshield is an extra 15 degrees steeper (I used to think this was way cooler), paint it more uniform for a 'vert (no black window moldings, etc.), different body kit, badging, tail lenses, rims, and obviously the back seat is a storage area which has a cover to hide the top when down. The top had a split 3-window design rather than the same one as the regular Mustangs did, and it also attached to the back of the windshield frame rather than the top like the regular Mustangs. The interior was quite different as well.
I don't believe there were any performance modifications, as you've already noticed.