Gas mileage is the bottom line. All new cars have clean emissions. A Prius getting 46mpg highway is only marginally better for the environment than a Mini getting 40mpg highway. An early 90's Geo Metro that got 56mpg is better than both of them, assuming that it is tuned to run clean. If the car companies would sell appropriately sized engines in cars for the commuter, fuel efficiency would be much better. As of now, only the Insight really took advantage of an electric motor to maximize mileage, and that is a assist hybrid to boot.

I am pretty sure that the reason the Prius gets such poor mileage is because of the catalytic converter. The catalytic converter needs to be kept hot to function properly, so the batteries are not allowed to maximize fuel ecomony as well as they could. The EPA needs to modify their requirements for hybrids to eliminate the catalytic converter.