I'm glad to see I'm not the only one who got tired of Wheel of Time, Goodkind's Sword of Truth and the Anita Blake series. Most of the people I know who read them thought I was a horrible person for saying each of those series should've ended around book 4 or 5 (or in the case of the Sword of Truth, book 2 or 3).
Dragonriders of Pern was good until her son started writing, the quality plummeted fast.
Edit: And not a series per say, but John Ringo should keep his rabid politics out of his fiction. His early stuff was good, then he started to let his politics take over his stories and suddenly he sounded like some of these bat poo insane talk radio guys.