Style has to do with how a piece of writing sounds. Everyone has a style which develops along with their writing; groups of people also have styles common to the group. For instance, if you read all the articles in Time magazine, the style is similar even though the authors are different. (This also has to do with audience, not to mention editorial policy.) Different subject areas require different styles. As for mechanics: Grammar, punctuation, spelling, and so forth certainly matter, particularly in terms of seeming credible to your audience (What would you think if you found repeated misspellings in your textbooks, for instance?). Style and mechanics are things to focus on only if you've already adjusted the content of your paper to your satisfaction.