I was thinking about this earlier after some tourists asked me the way to Camden Market (admittedly not that popular a tourist attraction but nonetheless I'd imagine they had a mildly disappointing experience).
So what's your least favourite 'must-see' tourist attraction? I think mine's Las Ramblas. When I was going to Barcelona everyone was telling me I had to see it and, whilst I loved Barcelona, I thought it was pretty much the worst thing about it. It largely consisted of long over-crowded street of people trying to flog you stuff, at least 300 counts of what would in the UK be termed animal cruelty and vague caution about the pick-pockets that loads of other tourists had warned me about. Some of the street performers further up are pretty damn impressive but I still didn't feel it was quite enough to redeem the overall shitness of the experience.
So where didn't you enjoy?