I hate:
M/AMG/RS/etc badges which don't belong there
Any body kits with the exception of factory options and subtle splitters etc
Tinted rear lights
Fly-eye headlights
Numberplates with stupid font
Incorrectly coloured headlight bulbs - this annoys me
HID's in reflector type headlights - but this one really ****** me off, and it seems to be quite common where I live now. 
Cars which are lowered so that the arch overlaps the tyre
Extreme camber
Excessive tyre stretch
Stickers which reference things which the car doesn't have, or which are tacky and not used ironically
Black wheels except in very specific car/wheel combinations
Halfords rear lights (you know the ones, Lexus lights and the like)
Poorly re-trimmed interiors / leather everything etc.
Chavtastic car audio incl custom plastic speaker enclosures etc
Air ride (though I sort of understand the fun in it)