Battery technology that takes enough charge so that you can drive a full-sized non-doofus-looking car at least 400 miles at 80+ miles per hour. (The Toyota Prius hybrid sucks. Yeah, I know that is politically incorrect -- bite me).
Then, the batteries would have to be cheap enough that they do not cost a fortune to service and replace.
The batteries would have to be compact enough so that they take no more room than a gasoline engine, transmission, and gas tank.
There will have to be a recharging infrastructure that is convenient, including for apartment dwellers. Recharging should cost no more than fueling up a gasoline car to go the same amount of miles.
The initial and total maintenance costs of an electric vehicle have to be no more than of a similar size and performance gasoline vehicle.
BTW, if it takes government subsidies for electric vehicles to "compete", then the costs are actually more, and we will all pay more in taxes and/or reduced "services" in other areas. TANSTAAFL.