I have tried reading Don Quixote and gave up about half -way through... it just couldn't keep my attention and I found it rather dull. I understand why it is a classic, but I guess I can't understand why two very dear people to me note it as their favorite book of all time...
Also, for some reason, I couldn't get through One Hundred Years of Solitude, though I really like Marquez and have read a few of his other novels. I do plan on trying this one again, however (as this happened with Madam Bovary a long time ago, and then I tried rereading a year or two ago, and I not only found it to be poetic and amazing, but I couldn't put it down).
And Nabokov's Pale Fire... I found my thoughts wandering every few lines... Again, I plan on re-trying this one.