Ahh - a travel review site here!
Let's see - what disappointed - well, not too much, and I've been to quite a few places. To me the key is to do research ahead of time in many different travel books and find that which interests you. For example: Florence can be amazing (David is breathtaking) as it is considered the heart of the Renaissance so admiring the architecture and art should be top on one's list. However, if that doesn't interest you, then you will be disappointed. For me, many times the food I eat or people watching becomes a big part of the experience. I love to go into grocery stores around the world - first to see what they do buy, how they buy it, how much they have, etc. You can get some great offbeat souvenirs and gifts from local grocery stores and usually at better prices than the tourist trap shop.
What I didn't like - Venice - was flooded when I went - the best part was one morning when I woke up and walked around the whole island alone just getting sort of lost. Naples - dirty city - best part was the museum which had all of the artifacts from nearby Pompeii and Herculaneum. Glasgow - we preferred hiking around Edinburgh. Dublin - I'm not a pub type of girl - and when wanting to see the Book of Kells (sp?) realized they only showed a few pages of the illuminated manuscript - could have just bought the book and seen it and not paid for the poor museum. Best part of Ireland is we found a booklet on 'local' restaurants and B&Bs - had some of the best seafood I've had in a small pub in Kerry, Ireland. Disappointed in the ruins of the Cistercian monastery in Ireland - realized I had seen a full working one with all of the gardens, etc, in Germany - better experience in Germany without realizing it. Big one next - Paris - this may be a case of 'been there, done that, many times.' Most of the big cities are fairly similar around the world - yes, the Louvre and Musee D'Orsay are cool, as well as the Eiffel (and really neat to me the L'Infermidad (?) - military museum), but after awhile, it gets old - you've seen it. Most of the restaurants are over-priced and the best ones are small ones in the outlying areas that the locals know about - oh, my only other two places I do love in Paris is the Musee Du Moyen Age/Du Cluny - has the Unicorn tapestries in it as well as ruins of a Roman Bath and Chapelle Cathedral (located near Palais du Justice and Notre Dame (did you know underneath the Notre Dame are more Roman ruins?)).
Cool places in France - Rheims, Epernay (can anyone say awesome Champagne?), Strasbourg, Alsace in general, Provence (Nice and surrounding area).
Other places over-rated - Napa, CA; San Francisco, CA (except for a few restaurants); Hollywood, CA
I did like Prague, but I wandered around the town and had a guide who had lived there. I didn't much like Warsaw, but loved Krakow. London was OK, preferred Leeds Castle and Bath. Loved Croatia - all of it. Loved Siena, Orvieto, Trieste, hiking/skiing the Dolomites in Italy, hiking the Cinque Terra in Italy, Santorini in Greece, Turkey for the sheer difference in experience and history, but didn't like the haggling and hassle; Morocco for the experience, again, but not the persistent requests for baksheesh (sp?); Phillipines was interesting - beautiful countryside, Manila a pit; Okinawa, beautiful, clean, gorgeous beaches, awesome seafood, really neat aquarium; Chile, Santiago, Valparaiso and Colchuaga wine region - also, snow-skiing in Andes; great place in general - prices awesome, food awesome, scenery and things to do all over the map - thinking of living there part-time when fully retired; Ecuador - Quito, sad place being raped by the government - Banos, nice resort, third world-ish; Otovalo, interesting Andean town.
OK - have rambled enough - bottom line - if one has too high of expectations, usually they are dashed; education before-hand can align the expectations. Also, knowing what you like and searching for that can make for a better experience. Ex: when we went to Santorini on the cruise, I found a 10K hike along the atoll from Fira to Oia - easily one of the best parts of our cruise - however, we like to hike preferring that to shopping. Others might think that is crazy. Different strokes.....
Oh, lastly - cannot stand plastic America stuff like Disney, etc - even can't stand Vail as they try to be an Alpine village - after you've seen a real one, you wonder why they want to 'copy' it - don't they have something unique they can foster? Maybe it's because the America we know is so young in comparison - don't know, but I think trying to copy something when you can have your own style (perhaps an evolution or derivative) is silly. Example, when skiing at Winter Park, I remember a beautiful restaurant at mid-mountain called the Sunspot or something like that - upon entering a great log-cabin like building there was an enormous stone fireplace and then they used copper on the outside of the building as well along with antler themed lighting fixtures, etc. Also, they had some copper 'sculptures' on the wall. That is definitely western American - the huge expanse (BIG - the adjective used by Europeans for American - and all of the meanings as well;-)) and the rustic-ness - great place. Another aside - I went into a spaghetti restaurant in Trieste and they decorated the place with all things US western - pictures of cowboys and Indians and the western US, red-and white checkered table cloths, picnic tables - it was surreal to be there in Italy surrounded by those things I grew up with (oh and the pasta was great - have never had a bad meal in Italy)! And many of the Europeans LOVE the western-ness of America like that - it's so different from what they have.