All the useless different sizes. Like some of the hub sizes, seatpost sizes, pressfit bbs, 1.5" steerertubes and tapered steerertubes, etc, etc. Pretty much anything that the mtb industry tries to shove down our throats.

 oooohhh loking forward ti this one...Raise hell @mikelevy

 Mom: how'd you guys like to try mtbkng?

 Me...

 29 ers !