I think it should be the awareness of citizen. The awareness is the base of the green city, only if the people attain a great awareness will the green city be sustainable. Such as the likes of recycling, conserving water, the acts are driven by the awareness of people.
Another thing I want to point out is that the amount of money invested for green city should be sufficient. If its not sufficient, no constructive and practical measures can be done, such as adding greenery to the side of a road and etc.