A good ESN does it's chores, doesn't stay out late, or consort w/ unsavory cell phones (iPhones especially). A bad ESN drinks a lot and hangs out w/ BlackBerries and iPhones...
An ESN is the Electronic Serial Number or identifying number assigned to the device @ time of manufacture. This number is unique to each phone and identifies it on a network. A bad ESN is one that's associated with phone that's been reported lost or stolen. This is also referred to as a blacklisted device. Carriers will not activate a blacklisted device on their network. I believe devices can also be temporarily blacklisted if the user doesn't pay their bill.