It needs to take a standard 7 or 8 speed cluster in the center, allow either wheel to turn faster than the other for cornering, drive the slowest turning wheel, and allow for brakes near the cluster to keep them away from the tire slop.
Light weight is a definite asset.
I don't know how many of you on this list are also on the hpv list, but this might be of interest to some or all of you. This book is really a hoot.