Word Choice is the use of rich, colorful, precise language that communicates not just in a functional way, but in a way that moves and enlightens the reader. In good descriptive writing, strong word choice clarifies and expands ideas. In persuasive writing, careful word choice moves the reader to a new vision of things. Strong word choice is characterized not so much by an exceptional vocabulary that impresses the reader, but more by the skill to use everyday words well. That's the test.
Fluent writing has cadence, power, rhythm, and movement.
It is free of awkward word patterns that slow the reader's progress. Sentences vary in length and style, and are so well crafted that the writer moves through the piece with ease. Writing that is strong in conventions has been proofread and edited with care. Handwriting and neatness are not part of this trait. Since this trait has so many pieces to it, it's almost a holistic trait within an analytic system.