Well, I recently said something like I can easily drop a book if I don't care for the beginning. But I also know a lot who've stuck it out (even if the book, for them, is terrible), and then there are those that have just ONE book they just couldn't finish. The book doesn't necessarily have to be bad - there are plenty of reasons why you couldn't have finished it. You could have lost the copy, and your budget is too tight right now. Etc.
Let me list mine.
Perdido Street Station by China Mieville. Started it. Loved the writing, but it was too... much? I'm not one for those ultra-slow builds, and I really did have to reach for a dictionary often. I thought the concept was awesome, but in the end, I couldn't make it past page 100, I think.
Sharps by K.J. Parker. I actually liked the beginning of this book. The concept was pretty cool, and I liked the characters. But, once they reached the fencing tournament, things got a little confusing. Probably because this isn't necessarily the first book set in the world. It isn't a bad book, but I just got too confused near the middle.
A Shadow In Summer. I picked this book because of the cover. It reminded me of the Night Angel Trilogy, and I loved that. I have to say, the beginning didn't sit well with me. I like Japanese-esque worlds (fan of mangas), but it just didn't click. Stopped it at page.. Hundred and fifty. May try it again someday when I've got the time.
(Basically, all of Pornokitsch's favorite books - I couldn't finish them.
Ender's Game. Didn't click with me. Writing wasn't awful, but I just couldn't seem to muster the strength to read it.
Neverwhere. I actually lost my copy. It was good, but I'm too cheap to get a new one.
American Gods. This tale's more complicated. For me, American Gods is one of those books where... you have to be in the right mood. And since I've gotten it, I haven't been in the correct mood for it. I got to the first hundred pages, but haven't felt like continuing it. Not bad by any means, but I just don't feel like it, at the moment.
The Novice by Trudi Canavan. This is actually an interesting story. I think I was 9 (so this was five years ago) and my sister dared me to read Magician's Guild. So, I read it. SHOWED HER UP. I was... okay with it. I didn't care much for reading in general, so it probably wasn't all that bad. I just didn't like the idea of reading. Read the Novice, and I got stuck on the chapters of this one character... Who was basically just going to a different country for a huge library. Yeah, so I dropped that. May give it another try in the future.
Uh... I can't think of any more, though I'm sure there are loads more. For now, those are the books I couldn't finish. How about you people?