The main flaw is the inability to battle other players. The only battles are in the gyms and it becomes boring or too hard (the only gyms around me are protected by kids with way too much time in their hands and their pokemons are too strong for a filthy casual like me).
The same problem exists in Ingress as well if you live in even a moderately populated area. Ninantic's core issue (in my mind) is that the longer their games run, the less approachable they become. They haven't figured out a solution for Ingress, so I'm skeptical that they'll figure out one for Pokemon Go.
Granted, Ingress is a little more "hardcore" in the sense that it's not really focused at a more casual market like the Pokemon franchise generally is.
