Spare time to read - Blogging is not just about writing but also about research and reading. Read as much as you can to learn new things and ideas that you can experiment with on your own blog.
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.
Image speak - A lot of bloggers tend to ignore this, but the images on your blog speak a lot for what you write. Use good quality images and make sure they express your written stuff in utmost simplicity yet precision.