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.
Use simple vocabulary. Important is to reach their hearts not testing their literary skills.
An interactive style of writing will help you gain readers, but steer clear of SMS lingo if you want to be taken seriously.