Some ideas I've used so far for getting students to comment:
rate your mates: this is an idea from Ewan McIntosh (he has a fantastic blog on all aspects of blogging, podcasting and new technologies). once some student work has been uploaded (PowerPoint for example) ask students to post two stars and a wish/two rocks and one sucks for a bit of peer assessment
planning: ask students to post ideas on how to revise, or how they are going to tackle work over the holidays.
ask a discussion or even exam question and get them to share ideas
never-ending story: ask students to write a paragraph of a story and then supply three words for next students to use in their continuation paragraph
So talking of comments, please add your with ideas of how we could use this feature of blogs to get interactive with the students.