I think some of these comments are down to people visiting somewhere for a very short period and then only doing the most obvious, hackneyed, touristy things on offer. If all you can think of to do in London is to look at Big Ben, visit the dungeon, go on a sightseeing bus and visit Madame Tussauds, yes it's pretty shit and over-priced. Similar for Edinburgh and York.
If, however, you spend some time exploring on foot, visit out of season or on a weekday (so don't go to York for the Christmas shopping!), and check out some of the restaurants and walks recommended by locals, you will probably have a lovely time. Much cheaper too.
I remember visiting the Louvre in Paris the first time. We spent a few hours exploring the different rooms, and barely saw a quarter of what was there. Fantastic, and loads still to see on a future visit. Then we went to the Mona Lisa room, and were instantly hemmed in by hordes of tourists crowding round and taking photos of this one portrait (madness - you can buy a much better quality print in the gallery shop if that's what you're after).
Given how empty the other rooms were, I bet that the majority of visitors just go to see the Mona Lisa for 10 mins, nothing else, and then declare the Louvre to be a 'shit, crowded rip-off'.