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.
Make good use of media (audio/ video/ embedded tweets, etc.) in your articles. It helps in grabbing the attention of the reader and supports your text.
Write regularly, neither too often nor too rarely. Two to three blogs a week is a good frequency to maintain.