I personally cannot think of a place we've been or a trip we've taken that was disappointing. BUT there have been some unhappy experiences on excursions or surprises at places we visited:
Loved our Italian excursions, but I guess after years of hype and seeing pictures since I was a kid, the Leaning Tower of Pisa was a shock -- it was much smaller than I expected and was put off by all the vendors harping at us as we tried to get to the entrance. Loved (finally) seeing it, but was just surprised.
The "worst" port that we've visited and wouldn't care to see again was Acapulco. The divers were great and the tour we took up into the hills was beautiful, and we loved the fort (which was literally next to the ship), but when our excursion dropped us off in town to "shop" -- it was awful. The whole area smelled like one giant urinal and we couldn't wait to leave. The streets and park had trash all over and, honestly, we didn't feel safe. We grabbed a few souvenirs and got back to the ship. We've been all over Europe and the Caribbean and that was the only port that made us feel uncomfortable and unsafe.
The worst excursion we had was in Antigua where the tour guide was more interested in showing us her friend's pineapple farm (not on the itinerary), which resulted in her having to cut our visit very short at Nelson's Dockyard (our one-hour scheduled visit turned into 15 minutes!) It was awful and that was the one place my husband wanted to see. When we got back on the ship, there was a long line of people at the excursion desk of people from that excursion asking for their money back -- which we all got because she was an awful guide and really disappointed a lot of people.
Now our last very bad experience was in Montserrat in Barcelona. To make a very long story short, the tour guide was awful, lost people many times and gave people the wrong boarding time -- told some of us 3:30 and others 4 p.m. Well she waited so long for the other people to arrive, and couldn't find one member who had to take a taxi back in the end, that by the time we left up there there was no time to see La Sagrada or the Olympic stadium which were all included in our excursion. Another trip to the excursion desk with a line of people and we all got partial refunds because they were well aware that she left people behind too and were not happy.