What Makes a DNF Book?

What makes a "did not finish" (DNF) book? I try to finish every book that I am reading, mainly because I paid for the book and don't want to waste money. Some books are just too dull to finish though. I usually give a book 50 pages to get me interested and then I will reassess it.  If I took the book out of the library then I have no predicament. I stop reading. 

So, what makes a DNF book? Sometimes I am not in the mood to read at all and just want to work on my art. Other times my mood prefers a light read such as a cozy and other times my mood wants something different. Assessing how you feel can help a reader determine whether the book should be saved for another day. 

The next issue is whether to review the book or not. I try to be fair but have been known to write punishing reviews on occasion. If a book is not in a genre that I like, I will review the book and state that I don't like the genre. I will also say what type of reader will enjoy it and give an "objective" review of the plot and characters. If there is anything about the book that I like, I will add that to my review.  I know that many bloggers do not review books that they don't like. I think it is more fair to your readers to know your true feelings.  Folks who read your blog do so because they have the same reading interests that bloggers do. I believe they will respect your views on a book that you do not like.

How do you handle those DNF books?