I found this old idea for an assignment meant to get students to formulate ideas about what makes good blogging practices:

For Monday, find an article that interests you from the last four weeks of the NY Times. Use Google to find a dozen blogs that have followed up on the article or the events that inspired it. Post links to the article and the blog entries and discuss the types of responses, their range, their strengths and weaknesses, and their value to readers. Then for Wednesday post a list of your suggestions for quality blogging, based on your findings. Read and respond to several of your classmates as they offer their suggestions, and in the days following refine your list as you consider their ideas and experiences. For the following Wednesday, post your revised guidelines for quality blogging. Come to class ready to discuss your final version in light of these exchanges of ideas.

In other words, define blogging by observing the creatures right out there in the wild.