1. the canti hanger rule, if it's there you must use it.
2. no theme on the build, meaning stuff just doesn't match
3. single speed on non single speed frames, please stop it, it looks so bad.
4. defo the lock hung somewhere on the bike ruining the paint pisses me off.
5. defo agree with the riser bar on negative stem thing.
6. saddle bags, hate them.
7. miss matched tyres, meaning brand as opposed to front and rear
other than this i can live with most things, i don't mind substituting certain modern parts like tyres/rims/grips/bars as i see these as safety items that need to be in good condition, i don't trust 20 year old bars on some of the stuff we ride so i think it is sensible to use newer bars for example.
canti hanger rule is purely aesthetic. meaning i think it looks crap if unused!
i like seeing a theme on a build, matching groupset, brands, colour, era, whatever as long as there is a theme. i don't like the 'parts box whatever will do' build.
high end old school frames that people decide they want to build as single speed is awful. please just buy a specific frame with horizontal drop outs and leave the nice old frame to someone else who will build it properly!
not being able to justify your build, i know you shouldn't necessarily have to but if someone does come along and asks 'why did you do that?' then you should beable to defend the reasons for your choices.