I thought that Mrs. Yollis' video "How to Write a Quality Comment" was a nice little video using the students in the classroom explaining the qualities of a good comment, which are as follows: 

1. Compliment the writer in a positive and specific way.  
2. Add new information to the conversation if the information has not already been discussed in the blog or its comments.
3. Make a connection -  Blog about your own experiences to make a connection to the writer.
4. End the comment with relevant questions which will help get the conversation going and spark more conversations.
5. Proofread your comment to make sure you have proper grammar.  Do not use more than one exclamation point.

The qualities of a good comment as told by theses students are all sensible.