I have no idea what this group is about besides Bionicle, so I'm going to post a few tips.
1. Make proportions reasonable. For example. you don't want a MOC with arms that reach the floor. Keep the arms about as long as the legs, and keep height in check too. In addition, you don't want the weapon to be the same size as your MOC, unless it's mounted on a base or a vehicle. A good example of proportion is Tanma.
2. Distribute pieces equally. For example, you don't want extremely bulky legs and nothing on the arms. Similarlty, you don't want the left side to have a lot, yet the right to have none.
3. You don't want to have too many colours. For example, a MOC with 6 colours would look way to silly. 2-3 colours would look best; 4-5 colours are managable, as long as they are similar. Black goes with anything.
4. Keep a theme. For example, a MOC with bat wings and a scary mask would look better than a MOC with fairy wings.
5.Make the MOC match your theme. For example, if you want a ninja, make it sleek and thin. If you want to make a mech or tank, make it bulky.