Sometimes, people may think that good writing simply involves proper spelling, grammar, punctuation and so on. Certainly, that's a big part of it; you might have written the Great American Novel, but it probably won't seem that way if your book reads like an I Can Has Cheezburger meme.
However, truly effective writing--indeed, effective communication in general--goes a step further: It takes the person who will read the content (the blog, the webpage, the article, etc.) into consideration. Simply put, a good writer knows his or her audience.