1) Squeaky chains, annoying ticks and clunks, and chains rattling the FD cage (cross chained)
on someone else's bike.
2) People starting from a stop in a MASSIVE gear on a geared bike and getting up out of their saddle and grinding until some speed is achieved--does anyone know how gearing works???
3) Tempests in teacups...
4) Mismatching wheels--whether that be tires, spokes, spoke lacing, and/or rims.
4b) Modern Campagnolo Bora/Zonda/etc rims with radially-laced spokes in front (totally fine) and triple-parallel spoke groups in the back. It looks horrible, doesn't match, doesn't have any logic, and makes the bike look worse.
5) Clunky/inelegant lugs and their friend tig-welded frames.
6) Unicrown forks -
Nothing deflates desire for a frame in my tall size range like a derp-tastic unicrown fork.
7) The "orgasmically-good" and supple and transcendent 1,350mm diameter tires that can be ridden at 4 PSI and are just as fast on gravel as racing tubulars are on the road,
and yet can never be made to mount straightly to a rim AKA "Hot Dog Wobble" (pick the front end of the bike up, spin the front wheel, and look down at the wheel as you would if you were on the bike and the tire will wobble or kick out to one side and then back. This is after all the tire 'truing' and fidgeting, too.)
8) Ill-proportioned or ill-angled frames (sloping top tube or level), especially for taller riders. Or just 99% of Riv bikes...
Again, as @
gugie said, this is not a list simply of things that bug me, but bug me more than they bug other people.