1) The more modular the design the better. Being able to field repair is extremely important. Replacable engine mounts, wings, tail feathers helps speed up repairs between rounds.

2) Protection for rx, battery, servos and fuel tanks. The more that is protected, the less damage suffered during mid-airs and the more likely you'll survive a mid-air to keep flying in the round.

3) In SSC, wingspans over 60" decrease your roll rate, in Open B, the speeds are higher and wingspan isn't is much of a factor.

4) Large vertical stabs to decrease yaw problems when streamers get caught on the ends of the wings..

5) Keep exotic parts out of the kit. If it can't be bought at home depot, leave it out... Most people cannot afford to buy alot of kits for combat. They will buy one, then reproduce it to build their fleets...