I basically fall into the camp of “do what you want with your car”. Aesthetically at least.

Except things that are just annoying or dangerous to others.

So I guess things like:

• Ridiculously loud exhausts and pop-and-bang tunes just done for noise. If I get stuck behind you in traffic it drives me mental.
• Badly fitted lights like LEDs in unmodified halogen housings that just blind everyone. Also permanently on hazards as DRLs (they’re amber in the U.K. Lot of VW folks seem to wire them to be on (but not flashinng), but at a glance it looks like they’re indicating/have hazards on).
• Chameleon tints. I get why you’d do it, but I just find them distracting and you can’t see the driver at all. That sometimes makes their intentions less clear and you can’t even communicate with hand signals. They’re often coupled with the type of people who never indicate so you can’t even infer their intentions from which way they’re looking.