A lot of domain cognoscenti think geo is the next boom.
I think most of the same factors that go into regular domains are in play- keyword purity (for example, chicago.com versus echicago.com versus chicagoweb.com versus chicago-online.com, etc, etc)
The economic prospects of whatever the domain addresses in its longtail and what type of competition might be had over sharp, concise domains in that category. For example, ChicagoCarDealers.com would probably sell for more than ChicagoBuggyWhipDealers.com. ChicagoPetStore.com would obviously be worth more than ChicagoPettingZoo.com.
The overall demographic and economic trends of the location are a big one.
Is it a declining, post-industrial wasteland or is it an up-and-coming, cosmopolitan city? Is it a vacation spot people like to travel to (and thus, might be searching for it online) or is it some random mudhole in the middle of West Virginia that is only visited by fertilizer salesman on business trips?
Obviously, SanFranciscoFineArt.com would probably be worth more than DetroitFineArt.com.