Geographic, family in the area for support, feel (how much you like the people there!), town, and then there is always the way the school teaches and its buildings and opportunities there. You can buy a few books that talk about med schools and read them before you answer that.
Also on SDN there is a interview feedback part where people talk about what they like dislike. You can always seize onto that for some answers if you got nothing.  If you got nothing jsut research and find SOMETHING to put there at least. Make sure you keep a copy for yourself in a folder so you can at least remember what you put before you go to the interview too..  
Try to so show some enthusiasm in your answers if you can.