You are not a proofreader. Chances are also high that the rules you are certain are correct, are anything but. Split infinitives? Turns out they’re fine. So don’t waste your valuable time on something you are unlikely to be good at. Even if you are good at it, it is still a waste of your time. Academic publishing make ludicrous profits from free academic labor. They can afford to pay for proofreading. And they should.
You are not a typesetter. Reviewers have spilt rivers of ink demanding that authors follow a particular system (e.g., APA 6th). Worse, reviewers almost always demand that authors follow their own idiosyncratic interpretation of these rules. They shouldn’t bother. The publisher will strip an academic paper of these style and apply their own. They pay people to do this. Don’t waste your time. Does the author’s style, or lack of it, intrude on your ability to read a paper? Fine, say something. But otherwise leave it to the pros who will get the accepted manuscript.
You are not an acquisitions editor. That is the editor’s job. Your job is to determine if the article has sufficient scientific merit to justify publication. Your job is not to decide whether a paper will be highly cited, be a pivotal piece in the field, or be ‘important’.
You are not a co-author. Your job is not to make the author write a paper the way you would have written it. Your job is to determine whether a paper would not be out-of-place sitting next to the existing literature in the field. You can suggest stuff. But if the author does not want to do it, and it does not affect the merit of the paper, then back-off. Better yet, after you write a comment, ask yourself: “am I imposing my style on the author, or does my comment address an issue of scientific merit”? If it’s the former, it’s better not to include the comment at all.