One of the first books I never finished Heart of Darkness by Joseph Conrad. It was required reading for English class.
I learned that I could read the beginning, the end and the middle, and still answer all the questions the teacher asked and still manage to write the essays. (This lesson has been useful in subsequent English classes.)
Heart of Darkness, as I recall, is a short book. Certainly shorter than many of the multi-volume fantasies I loved to read. So length isn’t a factor in what makes a book hard to read.
But I don’t really know what the factors are that makes a book hard to read.
For me, such factors probably include: 
- Dense
- Not Interesting
- Strange language
- No Plot
- Offensive characters/Plot
1) Heart of Darkness might have been dense. Maybe. Probably.
2) I can’t remember how interesting Heart of Darkness was, but I’ve read boring books for class cover to cover, so I don’t think this was the most important reason why I couldn’t read it. Even though it was boring.
3) No plot will turn me off every time, but I am not sure Heart of Darkness is plotless. Not that I actually remember the plot. I don’t. All I remember is him meandering down a river. But I didn’t actually read the whole book, so maybe there was a plot.
4) Some of the book might have been offensive. I distinctly remember reading a comparison to creatures and then being in class and thinking: Oh those are black people! Yeah, offensive.
I think I gave up on the book shortly afterward.
Story Treasury 
I can read just about anything, including in 2nd language or topics I’m less interested in, and I agree with your list. Old English I find challenging, and also older poetry or what I suspect as poorly translated poetry. Books (and novel series) that are highly repetitive can annoy me enough to start tuning out.
Heart of Darkness is one of the books I barely remember, too.
I can’t! You’re lucky!
I think this is an interesting topic to discuss. I guess it depends on the reader and what they find offensive that can make or break their enjoyment of a book. Regardless, if the book doesn’t interest me I typically won’t stick around for long.
Me either! Unless a mean teacher forces me to read the book.
Yeah I think the first book I never finished was Slaughterhouse Five, which is supposedly a classic. It was just too boring for me.
I never even started!