If you are going to target retail business (I think you should) you should keep in mind that most people working retail are young and have other commitments (school, other jobs, etc). For this reason I think having employee-editable "availability" schedules would be a killer feature.
edit: I see that you have a "request day off" feature: is it possible to request just a portion of a day off? Think - "I have an exam that morning but can work at noon"