When you are designing a level, there are several important guidelines to remember:
Be careful with gravity. It is recommended to put an arrow next to the gravity apple to tell the direction of the gravity, so that the player won't be surprised at the gravity change.
Speedloops, levels with very fast parts and big loops, are generally considered boring and unwanted.
Make sure your levels are suitable for the battle mode you choose, and not too hard (or easy, depending on the battle type).
If you are going to put the level up as a battle, choose the battle time with care, usually too much time is better than too little time. Use common sense though; moderators can and will stop a ridiculously long battle on a short level.
Avoid placing two polygons too close to each other if the wheels are supposed to fit between them. If the space is too tight, unexpected behaviour might happen, such as speeding up the bike unrealistically fast or completely stopping it. The result depends highly on the FPS and thus is most unfair and undesirable.
Do not make invisible polygons. (zooming to 10000.00000 times and putting vertices very close to each other). Use this REALLY only when needed very much and when it doesn´t surprise kuskis.