I voted discover mechanic. RNG has always existed (draw, knife juggler, etc), but there's almost no reason to play around anything because of "created by." Devs have repeatedly stated that each expansion has a keyword, and they like to keep that keyword isolated to the expansion for flavor or whatever. Yet the absolute worst, Discover, lives eternal.
I miss having to budget my answers. I miss having to play around what I anticipate my opponent will do. Now every game seems like "who can highroll the highest/fastest?" I still do dailies, but that's about it.
Played as F2P since a little before Naxx, started paying around Mean Streets, and went back F2P after Uldum. Been Legend, 6 classes over 500+ wins, whatever. (The HK protest shit, and a lack of updates/content in Overwatch also factored into my decision to go F2P)