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.
Always acknowledge a comment on your blog and respond to the commentator, preferably at his own blog. Its great to reciprocate the gesture.
Blog for the fun of it and show character. It is immaterial how many write about a particular subject, it is how you write the matters.