Same as many others: Clan of the Cave Bear and Dragonriders of Pern both started with excellently fleshed-out settings, but after the first few books, everything of interest in the worlds had been explored, and the author was left trying to maintain the same level of interest with plot alone. Unfortunately, by that stage, the author had not only run out of interesting world aspects, but interesting plot. They got kind of weak and repetitive.
Dune was similar, except that the later books would have done better as a few short stories: there wasn't enough in them to be worth stretching into a whole book. They mostly just spouted the political philosophy of the author anyway (the son of the original author).
And I'll say it: Twilight. I read them very shortly after they came out, and had not heard of it in the media or anything at all yet. I thought it was about chess from the cover. A friend recommended it to me, and so of course I read three (or is it four? I forget) books in a night. For something about immortal superpowered vampires, it sure managed to be outstandingly dull, poorly written, full of paper-thin characters, and packed with very disturbing morals (beauty = good, budding rapist = romantic, hormones > actually learning to love someone, stalker = charming, etc). I mean holy cow. A few weeks later the whole "Twilight sucks" meme started, so I like to think I'm a trend setter.
I will never get back the hours I spent trying to get to the "exciting parts" I was promised.