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.
Presentation is as important as good content. Have attractive pictures, fonts, etc. on your blog to hook the reader.
Write with conviction. Pick a side and be bold. And if you're wrong, admit it.