Fizzle, the only criteria is that you make the ornament. The variety is fantastic and runs the gamut from simple to more time-consuming. One year I made a simple felt heart with a bit of embroidery and a button accent -- simple but got good comments from the group. The real fun is in seeing how many different ideas people come up with. We've had cross stitch, machine embroidered, wood, quilted, felt, and on and on.
You should be able to find past ornament swaps and see the pictures that were posted. The main thing is to have fun!