Beer - Agreeing with most of the other people who posted that it tastes pretty terrible. Granted, I haven't tried very many brands, so maybe I just haven't tried the right one yet.
Ketchup - Ever since I was just a kid, I've never liked the stuff, even to the point where the smell alone is enough to make me a little sick to my stomach. For most stuff that people put ketchup on, I usually just substitute it with barbecue sauce.
Most online multiplayer shooters (FPS or otherwise) - Just not my cup of tea, really. I don't think they're bad or anything, I'm just not a big fan of the gameplay. They're okay to play with friends every now and then, but that's about it. From what I hear, most of the competitive communities are generally unforgiving to "noobs" too, but I can't comment there, since I haven't experienced or seen it personally. Shooters that have a single player mode with a decent storyline, on the other hand, I'm completely fine with.
Mario Kart (or racing games in general) - People seem to love the crap out of this game, but I've just personally never been able to get into it. I guess it's just because I'm more of a storyline guy than a multiplayer guy (which may also explain my dislike for multiplayer shooters).
Music videos - I don't hate them per se, it's just that I listen to music while I'm doing other things, so sitting and watching the video just isn't entertaining to me.
Social networking sites - Nothing against the people who use them, but I'm just not into the idea of posting all of my business online for everyone to see. That, and I'm not really interested in hearing every detail about other people's day (I guess that's more geared towards Twitter). Still, I do realize that they have good uses as well, such as keeping in touch with relatives and friends who live far away, and for professional purposes.
Reality TV - Out of all the stuff I mentioned, this just might be the one I dislike the most. The majority of reality shows are either: a) showing the everyday life of some celebrity (or wanna-be celebrity) that I couldn't care less about, b) some type of competition for people to show off their talent and become celebrities, which I also don't care about, or c) a documentary type show about a specific profession. Out of the three types, the latter is probably the only one I can even remotely tolerate, since some of them cover relatively interesting jobs . As for the other two, I don't care about someone's *insert insane number of children here*, nor do I want to bother with all of the drama in the competition reality shows. I watch TV to take a break from reality, not to see someone else's.