First, the pack should have both a waist and a chest/sternum strap.  Second, it should have enough volume to carry everything you need, with a little additional room for anything extra you might pick up during the day.  Third, it helps if it is has some reflective parts for increasing visibility when running during low-light hours.  Fourth, if you plan on running in all weather conditions, it should have a raincover.  You can buy them separately if the pack does not have one built in.