Mechanics, of rubrics I've worked on and used, usually includes: grammar, formatting, and language usage. For comparison and contrast papers, specifically, it should have the language of juxtaposition: "similar," "different," "comparable," "equivalent," "analogous," etc...
Good writing mechanics reveals a paper free of most errors in grammar and usage; its sentence variety is implemented with rich vocabulary; transitions are present; it includes project elements as assigned; it is properly formatted (MLA – margins, etc).
Here's what our state standards have to say about Mechanics:
1. Use Standard English conventions in all writing, such as sentence structure, grammar and usage, punctuation, capitalization, and spelling.
2. Demonstrate a well-developed knowledge of English syntax to express ideas in a lively and effective personal style.
3. Use subordination, coordination, apposition, and other devices effectively to indicate relationships between ideas.
4. Use transition words to reinforce a logical progression of ideas.
5. Exclude extraneous details, repetitious ideas, and inconsistencies to improve writing.
6. Use knowledge of Standard English conventions to edit own writing and the writing of others for correctness.
7. Use a variety of reference materials, such as a dictionary, grammar reference, and/or internet/software resources to edit written work.
8. Write legibly in manuscript or cursive to meet district standards.