The comments are there for people who want to explain their downvotes.
The only thing I can think of is an AJAX <div> reminder for users when they cast downvotes:

Please consider adding a comment if you think this post can be improved.
After the first downvote, we can't say we didn't remind them, and honestly that's as good as it gets. Forcing a comment will end in even worse results.
edit: this is now implementedstatus-completed and live for users with reputation below 2000. It is shown on every downvote until you get to 2k.