We can think of several elements, all contingent on the particular "style" the author is using in a particular circumstance, for a particular audience and function. What are the "style" elements we see in a particular piece of writing?
An editor can try to maintain the "style" of the author when editing. The objective usually is not to impose a style, but to help the author to maintain her own situational style by making her aware of what it is, and as an editor conforming to it as much as possible. Bad grammar, typos, ill-considered tone, illogical punctuation and paragraphing are not a "style" except in special circumstances. Surely an author doesn't need help with editing a work when the author uses those kinds of elements and wishes to retain them.1. Level of diction (big words, Latinate words, slang words, foreign words)2. Sentence structure (long sentences, short sentences, complex, simple, fragments, parallel and formal)3. Punctuation (dashes instead of semicolons, full stops rather than commas or semicolons, exclamations)4. Paragraphing (short or long, for emphasis or whole thoughts)5. Tone (serious, silly, sarcastic, witty, lofty)6. Person (first, second, third; one rather than you or we)