The batteries? The motor? Plugging in each night versus going to a gas station in the morning?
All of the above, of course, but also the high levels of interactivity now available to electric car users, via their smartphones. Ford is one such company touting smartphone app integration, at the Consumer Electronics Show in Las Vegas.
Connectivity is becoming increasingly important in the automotive world, and several carmakers are striving to incorporate your real driving experiences with the virtual world. MINI Connected, for example, can display your car's data on your iPhone, and lets you link to social networking sites.
Electric car apps take things a step further.
The MyFord Mobile app will be able to log driving achievements with fellow Ford electric and hybrid users, and upload driving data in real-time via networks like Twitter and Facebook.
They'll be able to keep tallies on how much CO2 they save by not using gasoline, or compare range with other users.
One option, known as Value Charging (in conjunction with Microsoft) will let drivers of cars like the Ford Focus Electric to "set and forget" a time for their car to charge, to make best use of cheaper energy rates at off-peak times.
A deal with MapQuest will also give users a list of all the nearest charging stations, linked with route-planning software to allow Focus Electric drivers to plan longer journeys.
Maybe it isn't environmental concerns that will spur on electric car growth after all - instead, the potential to allow drivers to connect with their cars better than ever before could be the real appeal.