The biggest obstacle, in my opinion, to vehicle-to-grid (V2G) applications is distribution utilities. All utilities have comprehensive procedures for studying and approving connection of distribution generation resources to their networks (rooftop PV, small CCGT, etc.). These take in account the maximum possible output of the generation asset as well as how they will behave under fault conditions.
They're already quite stingy about approving customer-owned DGs (because there is little or no financial benefit to the utility) - I can only imagine how they'll feel about people connecting a generator they don't control, at locations and times they can't predict, with the ability to disconnect them arbitrarily and drive away.
In order for V2G to become a reality, utilities will need far more advanced software and metering infrastructure to perform real-time load-flow analysis of their systems and determine if/how to dispatch connected EVs. At present few utilities even have the metering infrastructure to enable that kind of intelligent control, let alone the supervisory control software.