The nature of the copyrighted work;
Content that is fact-based can be easier to consider for fair use than content that is very creative in its essence. Facts are testable objects and thus can be valuable for consultation in a scholarly analysis. Creative and artistic work, like photographs or paintings, is more owner-identifiable and thus may be harder to claim for intentions of fair use.
For example, quoting from a fact-based scholarly article gives you a much stronger fair use case than quoting from a novel, and showing a clip from an instructional video is a better fair use case than showing a clip from a movie.