What constitutes a monotonous writing style?

Answer 1

Using the same descriptors and tense, as well as short even sentences can be monotonous. No change, no spark and no life.

Answer 2

If you are writing a story, and all (or most) of your sentences have the same pattern, it could be considered monotonous.

For example, "John walked to the park. He ate an ice cream. He saw his friends. He laughed at their jokes. Later, John went home and watched TV."