Here is my list:
Dubstep: Okay, I get electric music because it can connect with melodies... Dubstep doesn't do this. It has some melodies, but the most popular thing about it just pure utter noise. I just don't get it!
FPS Genre: I get the fact that people like shooting things, and they love challenges... I just don't get this because it seems to be the same exact game across the board. The only difference is how the games look, their stories, graphics, and titles. I just don't get it =/
Slenderman: I get that- Screw it, I just don't get the appeal.
Reality TV: I live my life, why would I want to see other people live theirs? I would be happy if they replaced this abomination with game shows... Hell replace with boring nature/history documentaries, I won't care.
That's all I have for now. I'll either edit this post or make another one with more things that I don't get.