I actually have a goal to DNF 10 books this year. I often stick it out with books that just are working for me. Now, I'll get to "accomplish" part of my reading goals by putting the book down.

Then probably no on Poppy Wars. I found all the war stuff really heavy handed, and seemed copy-pasted from Wikipedia page on the Rape of Nanking

Hyperion is a pretty demanding book, imo. It has 7+ point of view characters, and it shifts back and forth in time as well. It involves time travel, which is always a bit mind-bending. It's also more literary than most sci-fi in that has lots of allusions to other literature - Greek mythology, the Canterbury Tales, and the poetry of John Keats, just to name the most obvious ones. Personally, I loved it, but it's no beach read!

ETA: Also, Hyperion and Fall of Hyperion are a single story split into two books (much like Lord of the Rings is one story split into three books). Hyperion just ends abruptly, and you really need to read Fall of Hyperion to get the climax and resolution of the story.

>she sometimes forgets that her characters are characters and starts writing monologues in her own voice

This is key for me. If there's a character whose only role is to declare the view of the author, I find it really boring and annoying. I DNF'ed The Island by Aldous Huxley because of this.