This is a fairly difficult question for me to answer, because I don't usually start off thinking I won't like a novel unless I have a good reason for that predisposition - and in those cases I usually don't read the book in question unless there is some fairly compelling reason for me to do so. In all of the cases I can remember where I anticipated I would not like a book, it has turned out that I actually didn't like the book when I read it. I didn't like think I would like, for example, Dark Dawning or PureHeart, and when I read them, I actually didn't like either of these books.
Most of the time, I find that I have the opposite problem: Anticipating that I will like a particular book and being disappointed in some way by it. This is because I like science fiction and fantasy, and my usual reaction when I see a new science fiction or fantasy book is to look forward to reading it. I look forward to reading books so much, that I have piles and piles of unread books because I acquire them much faster than I can actually read them. When I do read them, I am usually very happy about the experience afterwards, but once in a while a book I was excited about reading lets me down. It happens fairly rarely, but it does happen.
I guess the closest thing to a book that I didn't think I'd like that turned out okay when I read it was the Dragonlance Chronicles by Margaret Weis and Tracy Hickman, which is actually three books: Dragons of Autumn Twilight, Dragons of Winter Night, and Dragons of Spring Dawning. I'm usually not particularly excited by licensed fantasy or science fiction, and licensed role-playing game related fiction is usually the weakest of this type of fiction, so when I picked up the Dragonlance Chronicles it was mostly out of a desire to catch up on the lore of the Krynn role-playing setting. I anticipated a fairly bad series of fairly weakly generic fantasy books set in a fairly bland Dungeons & Dragons based campaign setting. While I won't say that the books were particularly good - they are a fairly generic fantasy story - they were somewhat better than I anticipated, providing a mediocre but passable adventure story. While I didn't end up loving the Dragonlance Chronicles, I did end up enjoying them more than I thought I would when I picked them up.
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