I think a lot of books are overrated, not because they're bad, but because they're touted as almost flawless. I'd say this about The Lies of Locke Lamora, A Game of Thrones and the First Law trilogy. All very good books, but none of them is the pure genius that they're sometimes claimed to be. It seems to come with the territory: there was a point where almost every other big fantasy novel had "The new Tolkien" written on the back.
Otherwise, the one that really springs to mind is Starship Troopers by Robert Heinlein. This is not because of the politics, but because the promise of the story - an exciting war novel set in space - is never fulfilled. Instead, we get 200 pages of a man listening to some weakly-argued stuff in boot camp. It's extremely dull.
I've read The Haunting of Hill House twice now - often seen as the ultimate ghost story - and it seems a very damp squib to me. But so many people think it's brilliant that I always feel that I must be missing something.