Uhh, god it's a tough one. Not really a part but bullshit sizes of things that make no sense and break compatibility. 26" wheel can mean like 50 different real sizes depending on how you measure 26 inches.

The fad over internal routing is also awful. Great for aero, and I agree on off road bikes having cables run down parts of the underside of the bike that could hit rocks, or snag branches or whatever is bad, but having the cable leave the brake and disappear into the bike sucks for easy mechanics. I can probably do it now, but I wouldn't have learned mechanics if that's what I had to learn on