I can't think off the top of my head but one thing that comes to mind is french onion soup. I don't like onions, like at all, can't stand the taste and texture. To top it off, bread in a liquid is just gross, it gets all soggy. Was at a wedding and they served that as one of the courses. Had to force myself to eat at least a good part of it to not be disrespectful.
Another horrible thing I tried is blue cheese. Nasty stuff, don't know how people like that. I did not even realize blue cheese is basically rotten cheese when I tried it, so that knowledge did not play an influence in whether or not I'd like it, I just hated the taste.
I'm a simple man, give me my burger (with normal cheese) and fries and stop trying to be fancy. Oddly enough there are things lot of people hate that I don't mind, like broccoli and olives.