Favorite book series that ended badly.

[Benni88]

David Brin's Uplift Trilogy. Three meaty books which I had to track down and purchase as they weren't at my local library and the ending was atrocious. I felt a little cheated really, I had enjoyed the books so much up until the last hundred and fifty pages when I realised there was no possible way that a satisfactory conclusion would be reached.

 

[Swyftstar]

I will join in on the Harry Potter and His Dark Materials hate.
Harry Potter was, for me, in the beginning about magic and adventure and friendship and joy. I know, sounds corny, but everything we ingest can't be heavy... but wait, then it gets heavy. Like really heavy and dark and it just wasn't enjoyable to me anymore.
As for His Dark Materials, it just dragged and got boring and tedious and I, like others, couldn't even finish the damn thing.

 

[New Frontiersman]

A Series of Unfortunate Events. I loved the books they were funny and interesting, but the ending felt rushed, none of the lingering questions were answered, and it felt so out of step with the rest of the series that I just felt very disappointed by the whole thing.

 

[Berenzen]

For those looking to ASOIAF for a happy ending, I'd recommend reading his other materials first, going by those, it's not going to happen. These can easily be found in Dreamsongs v. I and II.

OT: Sword of Truth, for reasons already mentioned, and I was worried about Wheel of Time while Jordan was still writing it, his last books were fairly slow, Sanderson has done quite well ending the series.

Also, I can't bear to read past Gauntlgrym by R.A. Salvatore, simply because he killed off everybody except Drizzt.

 

[Caveworm]

I would say Dune.

Mr Frank Herbert created a truly immense and epic universe but sadly died before he could complete it. However, his son and a chap who wrote Star War comics or something, J. Anderson, came along and complete pissed all over Frank Herbert's universe, believes, and philosophy.

One does not expect a new set of books to be as good as the original, very much like Tolkien's work that was finished by his son. However, the Dune books just...well, are just an abomination to the first set.