Grammar - Depending on the language you use, blogging just makes it imperative that you are reasonably good with your grammar skills and proficiency if not a master. Remember your audience is a mix of both; the ones who are experts and the ones who are not.
Courtesy costs nothing. Be polite when interacting with fellow bloggers and your readers and your courtesy will be visible in your writing.
Originality - Focus more on the originality of content and on a consistent basis. It is a habit you will need to cultivate.