As I was reading through the "Waste of time" thread it occurred to me that I have not had too many experiences that were a complete waste of time, but I have been to several places where I spent too much time which made me leave feeling a little disappointed. If there is another thread on this, I apologize, but I thought it would be an interesting addition.
For me it has been Bern - we spent 3 nights, but I think I would have enjoyed taking at least one of those and moving it to Interlaken.
York - We spend 4 nights, but I think 3 would have been better.
Llandudno and Northern Wales - This was our biggest blunder, but with good reason. We stayed for a week. We rented a car and drove to other small cities, but it was still WAY too long. However, we were staying in a friend's vacation flat for free - so I won't complain too much! We did enjoy Snowdonia, Conwy, and Caernarfon - but we could have seen all that in 4 days rather than 7.
Any others?