Interesting topic. Kudos to whoever started it.
Among the ones that I like that alot of folk seem to dislike are;
Tad Williams' "Memory Sorrow and Thorn": I have expressed many times that this is my favorite fantasy saga of all time, even better than Tolkien's LOTR. Yes, I know, blasphemy, but Tolkien's LOTR, as awesome as it was, just didn't blow me away like MS&T did. Alot of folks seem to dislike it since it's really long and dragging during the beginning to about halfway of book 1 The Dragonbone Chair, and I must admit I almost was tempted to put the book down myself, permanently, but I held on for just a little while longer, and I was completely blown away.
Tad Williams' "Otherland": This blew me away almost as much as MS&T did. People seem to dislike it for the same reasons they dislike MS&T; mainly the pacing, but once again, if you just hold on for a bit and be patient with it, you'll be taken on the thrill ride of your life. Also, I've heard people didn't like it because they don't like sci-fi mixed in with their fantasy. This isn't an issue with me, so it didn't bother me.
Robert E. Howard's Conan Tales: I love Conan! I can't stress just how much I love Howard's tales of him! In my opinion, Conan is the greatest fantasy/sword and sorcery hero ever created. I've known a majority of people that find the stories to be incredibly cheesy and juvenile, and it turns them off, but from what I'm now seeing, opinions are starting to turn around about the Saga of Conan. Maybe it's because he's being revived through popular video games, maybe it's because newer video game series such as God of War and Prince of Persia are bringing back the blood and thunder/ sword and sorcery style of Howard's tales, along with movies like Gladiator, The 13th Warrior, The 300, Pathfinder, and the upcoming 10,000 B.C. Maybe it's because more recent fantasies are reflecting their Howardian roots, as I've heard. I won't question it, I'll just be thankful, as the old saying goes.

And by the way, a new Conan film is confirmed for 2009, which is going to be more true to Howard's tales this time, and a Solomon Kane trilogy and a Bran Mak Morn film have also been confirmed for those of you who didn't know.
"Tyrants and Kings" by John Marco: I honestly don't understand why this trilogy is so underrated and why so many people dislike it. As one other here said, the books were so great, I was moved to email John Marco, and he wrote back, and he was one of the kindest guys I had ever talked to. He answered all my questions, even the question of whether he would like it to be made into a movie, to which he answered, if they ever get popular enough, he would like them to be made into anime-style films.
Weis & Hickman's "Dragonlance Chronicles and Legends Trilogies": There was a time when alot of people loved these trilogies. For some reason, and I don't know why, the approval rate has gone down.
R.A. Salvatore's "Drizzt Series": I'd say this series has a pretty divided view, it seems almost to be straight down the middle, almost as if you either like it, or you hate it. I, of course, am of the former. I'm just blown away by all the well-choreographed battle scenes, the adventure, and the just plain fun of it. It seems these days though, the opposing opinion is becoming stronger from those who think it's too cheesy and juvenile. Some folks just don't like too read books for the fun factor anymore, I guess.
Now to the books that I dislike that everyone else seems to like:
"The Belgariad" by David Eddings: Now I hear alot of you out there saying, "WHAT?! You must have it mixed around!" No, I realize that there is a strong anti-Eddings mentality on this forum, and I assure you, I'm with it, but even those who say they dislike Eddings will admit that they liked "The Belgariad". I'm sorry, but I didn't like even that. The first three books were allright, but at book four it all went to hell. Polgara, Belgarion, and especially C'Nedra annoyed the crap outta me! Belgarath the Sorcerer seemed like a powerful mage that could have set all wrongs to right rather quickly and kicked Torak's ass six ways to Sunday, but chose not to for a plot device. Sorry, but I just don't care for Eddings. Period.
"Lord Foul's Bane" by Stephen R. Donaldson:I'd always heard great things about this book and the whole Thomas Covenant series for that matter, proclaiming them to be fantasy masterworks. I picked up the two series in hope of a real treat, but I'm telling you, when I picked up and attempted to read this book, I got the biggest, most excruciating migraine I've ever had in my life!! The rape scene that most people are offended by didn't offend me one bit, instead it was page after page after page of intricate, almost OCD-like descriptions of blades of grass and trees, and an endless discourse on Covenant's inner turmoil. Now I don't mind inner turmoil and brooding in fantasy, Michael Moorcock did it excellently in the Elric Saga, as did Weis & Hickman with Raistlin in Dragonlance, but having it just go on and on and ooooooonnnnn..... **SIGH**

I wondered if it was gonna get any better, if there was gonna be an epic battle or something, and the only one I saw was the battle between Covenant and those goblin-like creatures ( I forgot their name, something like Viles or something or other?) which I had too big of a blistering headache to enjoy. I almost didn't make it halfway when I decided to throw in the towel. I got to the part where Covenant made it to Kevin's Watch. So please tell me, does the series get any better? Are there any epic battles and awesome magic use or is it mainly full of Covenant's brooding and endless description of landscape? I would really appreciate someone letting me know because if it gets any better I will indeed pick it up again. But I don't wanna bother if it doesn't.
Stephen King's "The Dark Tower Book VII": The final installment in King's epic magnum opus irritated and disgusted me to no end! I absolutely loved books I-VI, but thought book VII was utter trash. King pretty much threw away all the magic and mysticism and western/medieval elements previously built up in this. I gave up halfway through and chucked the book across the room in disgust. Later, having a rather morbid sense of curiosity, I guess you could say, I looked up the spoilers to how the series ended on Wikipedia and found that it was even more horrible than I could have imagined!

It was almost like King was giving a big f-you to his most devoted fans.