Huge Wheels and lift kits (Clown shoes)
Logos/branding on the car* (most often coupled with clown shoes)
Stupid colors or color combos (most often coupled with logos/branding)
Bumper Stickers of any kind... especially ones advertising that your kid is an honor student, or who the person supports in an upcoming election. Seriously? Nobody cares.
*Not to be confused with logos put on the car to represent race sponsors. However, if your car is not sponsored, and thus you get no money or benefits by putting stickers or logos on your car... then why make your car look like a giant piece of crap by loading it down with stickers?