I have to laugh about the Little Mermaid - the clue is in the name :) But the hoards of tourists around it are what make it annoying, not the actual size of the statue in my opinion.
I am very fussy about where I go away and don't go to places if I don't think I will like it, so I am not often disappointed, even if I also not often excited by places because I've been before - but knowing your way round and knowing you will like a place makes for a more relaxing holiday.
So I am rarely disappointed or think somewhere is overrated. There are a few places in the UK that people rave about and I wonder why; but usually when I don't like somewhere it's because there are too many people. I went to Prague and wondered what the all the fuss was about, went back six weeks later when it was much less busy (first time had been a bank holiday) and loved it.
I wasn't massively taken with Florence or Seville, but I wouldn't say they are overrated, just not my sort of place. Oslo is very expensive but I still liked it as a place, it would just be nicer if I were richer :)