I personally dislike bottle cages on mtb, we have hydration packs now. Less faff on the frames please!
Definitely no cable locks wrapped around the frame, or worse bar and stem.
No reflectors or bells. You should have lights at night and anyone can say “excuse me please”. Unless you are mute, in which case you can have not only a bell but a curly bulb horn too.
No stickers of brands you are not riding/wearing, there’s something tragically aspirational about it.
Fatter tyre on the front, tread in the correct direction.
I struggle with seeing barends, but moreso when they are the wrong way around. If you must, do so correctly please!
Please don’t pedal using the back half of your foot, it’s ugly.
Oh, and don’t let other people tell you what to do 😉