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.
Build a community - Connect with like minded people/readers to know what they expect out of you. Interact with them at regular intervals and take their feedback. It will help you gauge a trend and hint you towards a direction you want to take your blog to.
Content is the king - In the blogging world so diverse and big, you need to establish yourself with a distinction. the only road to it is good, original and valuable content. Be creative, different and surprise your readers once in a while.