Tags: Use of relevant tags is important from the SEO perspective. It helps the search engine track your post based on the tags you use.
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.
Write regularly; not too often or too rarely. Two to three blogs a week is a good frequency to maintain. Depends on the genre though.