I think "good" might be in the mind of the blogger. If you are blogging about something you are passionate about, for you it is a good blog. Chances are there will be others who feel the same way. I think a good blog should have correct spelling and be straight forward. Don't mess it up with "bells and whistles" that don't enhance what you are trying to say.
A blog could enhance my classroom by "knocking down the walls" and allowing students (and their teacher) be more self-reflective learners. Collective knowledge and encouragement could be shared and self-directed learning might be more likely. In a classroom setting, some students might be reluctant to share their thinking or opinion, but a blog might allow the reluctant speaker to open up. At the very least, I would have a peek into the mind of all of my students, not just the ones who were called on in class.
Absolutely. It is free and available almost anywhere. It might not get a lot of readers and it would be hard to make money on it, but the main purpose of a blog is not to make money, but to share your ideas and experiences with a group of like-minded people (or relatives).