In other jams you must rate a minimum number of entries if you want to be rated. For example, on TINS and SpeedHack they send you one or two entries you must to rate and you must rate a few more. If you don't rate that minimum (they're very small contests so maximum were about 4 or 5) the votes you receive doesn't count.
For example, a requirement might be to rate 10 games or so. It think this is fair and will encourage people to rate.