Matching headstocks on maple boarded basses. Pretty much the only thing I can’t deal with.
edit - and spiky basses.
There are quite a lot of things which put me off actually, but all silly little things and nothing definitive.
The Sandberg headstock always looks wrong to me, “stealth” satin black finishes which only look good for 10 minutes before the bass looks like a greasy chip bag and silly things like mismatched headstock design vs body shape.
Anyhow, variety is the spice of life etc.