Just to show you how views vary…I ran across “Startide Rising” and “The Uplift War” before I found “Sundiver.” I loved SR and especially TUW, which I’ve read several times, but I’ve started “Sundiver” perhaps four times, and couldn’t get past the first 50 pages.
The last few of the Wheel of Time books. What a smouldering pile of ass a promising series has become.
I liked Armistead Maupin’s Tales of the City books less and less as the series went on. By the end, Mary Ann and Mona had become completely insufferable, and a lot of the humor of the first two books was sorely lacking.
And the third book, Further Tales of the City, with the whole Jonestown subplot? Yech.
That said, those six books are still some of my favorites.
For me, all three sequels to Hyperion were disappointing, the last two especially.
William Goldman did a sequel to Marathon Man called, if I recall, Brothers. Bad idea.
Yet another vote for Rama II and Riverworld Throne, two unfortunate examples of the theory “write anything, we’ll sell the title”. But I did like Startide Rising.
William Goldman also wrote The Princess Bride which has deservedly become a classic. He wrote a sort of sequel called The Silent Gondoliers which has deservedly been forgotten.
On an obscurer note, Jonathan Nasaw was a minor novelist of very limited success when he wrote The World on Blood. This book is, in my opinion, one of the best vampire novels written in the last twenty years. Others apparently agree and it became a best seller. Nasaw must have felt the pressure of producing a follow-up and his next book was a sequel, Shadows. Unfortunately, lightning did not strike twice. Shadows went off in such a different direction from The World on Blood that I almost suspect Nasaw had not written the book as a sequel but then decided to cram the previous book’s characters into roles in the new book. Shadows did not sell well and none of Nasaw’s subsequent books has done well either.
2010, 2061, 3001. Made worse by Clarke’s liberal quoting of the previous books, sometimes as whole chapters.
Oliver’s Story, a sequel to the classic romantic tearjerker Love Story. Oliver became sympathetic as he found his own moral path (diverging from his family) and especially as his wife died in the first book. In his eponymous sequel, he is turned into a banal, pathetic widower who ends up in a relationship with a woman who is so much of an outrageously polar opposite to his dead, beloved, Jenny that you spend many pages of the book (really a novella more than a novel) rooting for his new love to fail. Ech.
3001 was particularly disappointing, especially the ending that echoed the movie “Independence Day.” To Clarke’s credit, he did try to justify such hokum, but I didn’t buy it nonetheless.
Ummm… anyone remember “Closing Time”, a little dogs breakfast of a sequel (in the loose sense) to a little book called “Catch 22”.
I think this “Closing Time” actually defines this topic.
40 minutes. You beat me by 40 stinkin’ minutes, cankerist. 
Closing Time isn’t anywhere near as good as Catch-22, but then, what is?
Another disappointed sequel I just remembered: Aztec Autumn, the late Gary Jenning’s mediocre sequel to his bestelling Aztec.
Another vote for Hannibal.
Loved Red Dragon, loved Silence of the Lambs, absolutely, positively despised Hannibal. Just a huge stinking pile of crap.
And Mr. Harris, if you’re tired of the characters, then don’t write the friggin’ book.
The Bloody Red Baron, the sequel to Kim Newman’s Anno Dracula. The latter was an interesting, fairly well written take on the Jack the Ripper story, involving characters from a myriad of Victorian stories and literature. It was a really fun read. The former attempted to duplicate the idea by continuing the story in to WWI, but the character cameos and such overwhelmed the book, and a lot of the “old” characters were completely different acting than they were in the first book.
I, too, hated Hannibal/loved Red Dragon. Also, add me to the list of grumbling villagers ready to storm Robert Jordan’s castle!
The Black House.
I’ve never been into the whole Dark Tower thing with King, but I was hugely disappointed with Black House as it had only the tiniest thread connecting it with The Talisman and most of the book consisited of description, rather then action. Which wouldn’t have been so bad, if the description had been of something that was even vaguely interesting/exciting.
Not a chance.
I read the Rama series in a row a number of years ago (pre-kids), and enjoyed them very much. So everyone’s wrong there.
Larry Niven’s lost his touch, IMO, and the Ringworld series dipped after Engineers.
But the worst sequel and the LONGEST FUCKING WAIT definitely goes to Hannibal. coyasicanbe got that so right. Mr. Harris, please retire if your characters bore you. Bad baooks don’t so anyone any good.
When I was a kid I started all the Dune sequels. I read a couple, I forget which ones, never understood anything. Might go back now that I’m growed up and all.
Oh yeah, Shelters of Stone was really bad.
There are two that top my list of bad sequels:
Diane Gabaldon’s Fiery Cross. I waited a looooong time for this sequel to come out and it was so disappointing. Gabaldon’s sense of humor vanished! I had to read 200 pages before anything of note happened. I think it’s time to wrap up this story and these characters.
Lonesome Dove’s sequels, like Streets of Laredo. LD was an incredible book. McMurtry should have left it alone.
Mary Karr did a brillant job of treating a troubled girl’s early childhood in The Liar’s Club.
With great anticipation, I ordered the sequel, Cherry, which dealt with the same character in her teenage years.
I finished reading Cherry, but compaired to Liar’s Club I found it to be weak and without meaning.
Okay I have one more. It’s not so much a sequel but I read Small Gods by Terry Pratchett and I really liked it. Later I tried to read his other work and just put it aside because it was unreadable. Two other Pratchett novels later, borrowed, and I still can not read his work without putting it down.