I can't believe I haven't already replied to this, but anyway, here's a long list of overrated books in fantasy/science fiction:
1) The Wheel of Time by Robert Jordan - already mentioned and fully discussed earlier.
2) The Farseer trilogy by Robin Hobb - yes, its good, but the last book was pretty poor, with one of the worst endings ever to a fantasy series. Yet people still say its the best series in fantasy. I don't think it even has a strong claim to being the best epic fantasy today, with such strong competition, and its best part, the characterisation, still isn't as good as the best in fantasy.
3) Lord of the Rings by JRR Tolkien - you all knew it had to come some time. Sure, it's a good series. It's an amazing feat in worldbuilding, pretty much unparalleled in depth. But is it a good story? The plot, characterisation, prose - all are sketchy at times. Personally, I don't think it can compete with either its contemporaries or with modern epic fantasies. It was a children's novel that could appeal to adults just as easily, but it wasn't a masterpiece. The masterpiece of the time, Gormenghast, was hideously overshadowed by this.
4) David Eddings - anything he's written. Hugely popular, hugely derivative, boring and repetitive. He's honest about it, but the novels are just about passable when you're first coming into the genre and slowly degenerate after that.
5) Wit'ch series by James Clemens - over-rated, not because of its popularity, but because of its sheer awfulness. It's definitely the worst fantasy series I've had the displeasure of reading, ranging from idiotic characterisation to a perfectly predictable plot to cliched worldbuilding to ridiculous apostrophes in every other name.
6) Foundation by Isaac Asimov
7)Dune by Frank Herbert
Both of these, unlike most of the others, I really enjoyed and found them to be excellent. So why did I put them on an over-rated list? Because both have acquired a similarly mythic status to Lord of the Rings in fantasy, and this overshadows the true masterpieces of science fiction, such as The Forever War (I would have said PKD, except I don't think PKD can be overshadowed).