All of the above.
Think of it this way . . . you pretty much need the same qualifications that you'd need to get into any other school at Northwestern, but you also need an uber-impressive essay (to show those writing skills) and prefereably a slew of journalism related activities and/or leadership experience. Although, that's not exactly necessary - on the facebook group for Medill '11, for every editor in chief, there is one kid who has no journalism experience but was interested in the major anyway.