Commercial vs. educational nature of the use. The judge is more likely to rule in your favor if you used the copyrighted material for educational purposes rather than to promote a product or service. Judges do understand, of course, that almost all publications have some commercial or promotional purpose, and your use of the material will fall somewhere between commercial and educational. Fiction vs. non-fiction. It's generally fairer to use factual material, especially if it consists of public-domain information, than fictional material.
Courts recognize that the public has an interest in the dissemination of important facts.
The amount used. The judge will compare the amount and substantiality of the portion that was used with the entire original work. The less you use, the better. The best policy is to use no more of the work than necessary to make the discussion accompanying it comprehensible. Effect on market value of the original work.