What's the most important thing to keep in mind when blogging?
Your top priority should be Provide Value.
Information value. Entertainment value. Infotainment value. You are either expressing yourself narcissistically, with no concern about the impact on, or reactions of, readers. Or you're trying to benefit an audience, in some way.
You've already decided what your blog is about. You've determined what your expertise, art, skill, talent, interest, hobby, passion is.
You may have even some idea of how you would like readers to respond to your blog: post comments, email suggestions, vote on sidebar poll, submit nominations for an honor, download your music mp3s, view your digital art, or perform some other interactive, participatory behavior.
Don't focus on what you want your readers to do, think, or feel.
Too often, we get pre-occupied with behaviors we want our message recipients to manifest, for us, for our goals, for our ego. Instead, shift your emphasis and your creativity on what might benefit your audience, on their own terms, not yours.
Concentrate on what you can provide, on the content and presentation of text, photo, audio, video, etc., that will benefit your readers. Is it what they requested, need, crave, like, prefer? Is it presented as effectively and professionally as it deserves?
Inspire. Inform. Instruct. Warn. Scold. Joke. Amuse. Shock. Entertain. Reveal. Encourage. Provoke. Confront. Challenge. Elevate. Improve. Transform. Astonish. Delight.
Do that for your audience, whatever they have told you they need, or whatever you feel you do best, or whatever is most timely and relevant to them.