This is a great thread. Think mine have all been mentioned.
Venice - obviously, it’s amazing but I had a bad experience and was shouted at (unfairly!) by some Venetians. St Mark’s Square was overcrowded with huge tacky advertising hoardings, and cruise ships would stop discharging huge numbers of tourists. Got dragged round too many churches.
The Lake District - so glad someone else has said this! Dull little towns and nothing to do. I’m probably biased as I’m from the Peak District which I think matches it for beauty and has more interesting towns and villages. I think Castleton, Eyam, Matlock, Bakewell, Buxton and Chatsworth beat anything in the Lake District. My in-laws though are Lake District nuts and so there are many family trips there.
Montreal - to be fair, I wasn’t desperate to see Montreal and was visiting a friend but it’s like a second rate American city crossed with a second rate European city. Quebec more widely also seemed very dull. Just lots of pine forest. Quebec City is quite cool though.
Dublin - agree it’s just a bit meh.
Places that have exceeded my expectations:
Prague
Munich
Belfast
Washington DC
oh, and we did San Francisco about 14 years ago and found it fine but that was before it really headed downhill. I expect all cities to have grot spots and dodgy bits but I don’t remember feeling particularly unsafe. We did venture into a Latino area a bit off the beaten track to eat at a particular taqueria and kept our wits about us but nothing bad happened.