1. All of the lights people mentioned before.
2. Stickers for products you don't have.
3. HUGE aluminum wings(I usually thing the smaller more conservative ones look ok).
4. Unpainted body panels
5. Steelies/donuts on the street
6. Dropped cars(sure you've all seen those...The style of lowering where they push the wheels out further from the body and make the car looks like an RC car only not as good and a whole lot GHEYER).
7. Bad interior paint jobs. If you can't afford to get it done by the pros you're probably better off not doing it, b/c 90% of interior paintjobs chip, fade, peel, and flake and look extremely bad.