1, Make a sex wagon - Chicks dig sexy cars.
2, Make it fast - All show and no go makes you an unemployed designer.
3, Not going up in flames after the first few races is a plus.
There is very little choice the designer has when choosing a shape for a modern F1 car. The FIA marks areas around the car that are not allowed to have any bodywork in. That's why all cars look so similar each year.