A good blog must be organized, have a clear topic, stay on topic, and must be updated frequently.
The reason why people blog is to exchange ideas or express what the blogger is thinking.
http://www.intuitive.com/blog/why_do_people_blog_how_many_bloggers_are_there.html
http://www.articlesbase.com/blogging-articles/why-do-people-blog-834110.html
http://zedomax.biz/blog/blogging/what-is-blogging-why-do-people-blog-everything/
http://www.43folders.com/2008/08/19/good-blogs
http://jakonrath.blogspot.com/2006/03/what-makes-good-blog.html
http://www.webpublishingblog.com/what-makes-a-good-blog-post.htm
http://www.lifehack.org/articles/technology/what-makes-a-good-blog-post.html
http://vandelaydesign.com/blog/design/what-makes-good-blog-design/
http://www.blogadviser.com/what-makes-a-good-blog/
http://www.findableblogs.com/what-makes-a-good-blog-post/