I, somewhat masochistically, stuck with The Wheel of Time series long, LONG after I should have quit on it, but finally stalled out in the middle of Winter's Heart. I never finished it and will never read any of the subsequent doorstops in that series. I do try to be loyal to authors I like, and I realize Jordan was dying, but.... please.
The Sword of Shannara was the first book that I ever seriously considered chucking in the trash. I found it in the day room of the barracks at summer camp in ROTC and the only reason I didn't was because reading material was in short supply and it didn't belong to me to start with. Total Tolkien rip-off.
I read one book by Mercedes Lackey, having bought it sight unseen through the Science Fiction Book Club. I don't remember the title, but I will never, ever, under pain of torture, ever read anything by her again. I can only surmise from her popularity that it was an aberration and her other books are better. I leave that to someone else to decide/defend because huh-uh, no way am I going to waste money on an author who not only spends the entire book telling and not showing but then tells you twice what she just told you. Please, authors, do me the courtesy of assuming I have a brain and know how to use it.