Probably this. Was about to say Segwit and the day I saw bc-- addresses but this definitely took me to a rather poor memory of my first searches for bitcoin. I was, for the most part of 2013 (especially July) having a hard time storing all private keys and determining which is which. Perhaps that was one of the reasons why I left bitcoin and returned in 2014 where everything was relatively easy (at least for me) to operate.

Segwit also takes a spot due to the drama it has generated since its initial documentation. It also took a couple of months before services and other explorers implemented the new format.