This has been quite an interesting thread to read. I actually enjoyed a lot of the books that people have listed here (despite having studied some of them), like Catch-22, The Great Gatsby, Emma, Frankenstein, The Catcher in the Rye, The Lord of the Rings, 1984 etc.
There are however a few books that I haven't finished, not even because I disliked them, but because I just never got around to them. The two that immediately spring to mind are 'Invisible Man' by Ralph Ellison, and the Gormenghast trilogy by Mervin Peake. I recall actually enjoying them, getting maybe two thirds of the way through the former and halfway through the second book of the latter, however evidently something must have come up - maybe I got a new book that I was more immediately drawn to, or maybe I misplaced them for a while, but when I rejoined them I couldn't recall what exactly was going on and I steadily lost interest from thereon in.
Something like that actually happened to me a while back, when I was reading 'The Count of Monte Cristo'. I stopped reading it and couldn't get back into it, however one summer when I had a lot of time and not a lot of things to do, I picked it up again, got right into it from the get-go and finished it in a few days. It's actually one of my favourite books now (purely by merit of having an incredible story).
One thing I find - and I realise that I'm rambling on a little here - is that I enjoyed these books far more in hindsight, when I look back and reflect upon them. That's why I nearly always try to finish books even if I'm stuck in a fairly boring part, as often the whole is greater than the sum of its parts. To take a more recent example, over the summer I read the book 1Q84 by Haruki Murakami. That book was looooong (like 1000 pages or so) and a lot of it felt slightly esoteric (which I suppose is in keeping with the plot of the novel) and deliberately mystifying, which could have led to it feeling lazy with a kind of manufactured intellect (which is slightly how I felt about The Road), however once you reached the end of the novel, and its deeply satisfying conclusion (which I won't spoil) it felt like a lot of the strands (though not all) were drawn together and one (or at least I) was left feeling like it had all been worthwhile. Obviously this is very personal to me, and I'm sure there are some people out there who both read it and hated it but I guess that kind of sums up my ethos with that kind of literature.
That's not to say that there is nothing that I've disliked or found boring, which I guess is kind of the point of the thread, but I guess what I'm saying is that even when I've found a book dull and uninteresting I've still usually managed to take something from it that made it worthwhile.