The Needles - Isle of Wight. Just hideous, can't believe we queued in a traffic jam to get there. Horrible tacky theme park. Our fault for not doing our research properly.
Mont St Michel too. Glad we went because I don't have to wonder anymore, not actually unpleasant but not really magical. Too crowded and commercialised. Prettier from a distance.
Sad to see Turkey mentioned several times. I used to live there and thought it would be hard to dislike it as a holiday destination. I don't like Marmaris though, very down market and depressing. It's a long time since I've been to the south, but there are some beautiful places.
Unlike most posters here, have had a good holiday in Tunisia, but went as a couple (husband is muslim) and I speak French. Travelled independently, did Tunis and Djerba Island. Djerba is nothing special but we stayed in a lovely French run guest house. No one on the island was at all threatening or pushy. I can imagine going to package holiday resorts with DCs would be a different experience.