Les Miserables by Victor Hugo. Over a thousand pages of beautiful French literature, but the page count is some serious. It's intimidating.
The Way of Kings by Brandon Sanderson. I love Brandon Sanderson, but this is an epic high fantasy series I have to mentally prepare myself for.
Mrs. Dalloway by Virginia Woolf. I have never been a fan of stream of consciousness so this novel would be difficult for me to follow.
The Discworld Series by Terry Pratchett. It's not so much the content of the books as the length of the series that is intimidating. Over 40 books! That's a serious commitment.
The Luminaries by Eleanor Catton. It's just a huge book and I've tried to read it before and was completely unsuccessful.
Don Quixote by Miguel De Cervantes. Must I even add a description for this one?