In regards to WHS not worth visiting, well I guess to me they are all worth visiting.
My holiday plans may boil down to visiting some places that some wonk in a Department of Cultural Affairs thought had a good shot at getting inscribed, but usually the sites tell an interesting story.
Having said that if I wasn't so interested in ticking off the list these are the ones I found most underwhelming:
Dutch water management sites - they tell a unique story, but my my they can be dull for the non specialist.
Derwent Valley Mills - These could be rather interesting, but in reality you end up trotting around a series of disused factories turned into shops selling lavender soap, shortbread and wool sweaters. The UK seems to really struggle with the presentation of its industrial WHS.
Mir Castle- once we got past the pretty exterior moat this castle was so devoid of merit it actually gave us a bout of the giggles, nearby Nesvizh looked magnificent in comparison. There is a reason Belarus isn't topping many must see lists (Minsk has provided many years of worthy stories though).
Ming Tombs- don't go as part of a combined tour with the Great Wall, a very frustrating experience, though I guess they are more interesting if you get there under your own steam.
Hovgården- didn't really do much for us, the island it was on was nice in the midsummer sun, but on a dark damp day I think this may have scored very low.
Corvey- I didn't get inside but Els and Solivagant's recent reviews certainly don't make me want to rush back.
Donana NP- all I saw was a horse from 600m away in some scrubland, but the boat trip was nice and the
Manzanilla was even better.
European Palaces- there are some that really stand out, but there are far too many that are expensive, involve limited visiting hours or endless guided tours that seem to revolve around highlights such as a painting of the 12th Earl of Borsetshire's hunting dogs, an immensely faded tapestry or a collection of Porcelain serving dishes. Hmmph.
Of course I still aim to visit them all even if there are joys like the Royal Exhibition Building or the Chestnut groves of the Ceveenes awaiting me.