Almost all anime, but especially any sort of anime that is supposed to be a comedy. Every anime I've ever seen that is clearly supposed to be funny just feels cringe-inducingly dumb, like watching a terrible stand-up comedian.
Star Trek. The old movies (and show) were just painfully slow and boring, while the new movies went too far in the opposite direction and seem made specifically for people with ADHD.
Toys. I get the concept of 'collector's items', but I'm sorry, they're just toys. You can slap whatever label you want on them, they're still just toys that you don't actually play with. To me, that's like people who buy $200 sneakers that they never actually wear.
Dr. Who: I give the show a lot of credit for being very creative, but I remember watching it and thinking "I don't get it, are the special effects supposed to be cheesy and atrocious?". I tried a few episodes, but it just never clicked for me.
Bronies: You can like whatever the heck you want to like, I don't care, but for some odd reason so many Bronies I've met take this strange and borderline arrogant pride in it, like they 'get it' and the rest of us don't. It's a show aimed primarily at seven year olds, it's not that complex.
LARPing: I don't generally say anything is just "too geeky" for me, but LARPing. Yeah, too geeky for me.