There really is no excuse for writing poorly in one's blog. At least with IMs and e-mail your terrifying disembowelment of the language is limited to one observer. But in your blog, you'll look stupid for the whole world to see, and it will be archived for as long as humanity remembers how to produce electricity. Maybe you don't think anyone who reads your blog will care. But I read your blog -- yes indeed I do -- and I care. Madly. Truly. Deeply.
John Scalzi is fast becoming one of my favorite writers (see Old Man's War). Here he presents some useful tips for aspiring writers.