Back of the envelope/discarded printout.
The biggest problem this thing has is battery power.
I think an issue of EE times once claimed that gasoline has eighty times the energy density of a good lithium-ion battery. When I worked it out on paper (comparing gasoline, at 32 MJ/L and about 0.72 kg/L) to the Tesla Roadster's battery (190.8MJ at 450kg), I get that gasoline is about a hundred times better; however, much of the mass of the Roadster's battery pack is wiring and other materials, so that number has to be high.
Let's be generous, then, and say that (a) Lithium-ion battery technology has gotten a lot better and that (b) only half the mass of the battery pack is batteries. A stretch, but a good upper limit to energy density: Gasoline is somewhere north of fifty times the energy density of lithium ion batteries. So here I've derived the obvious: The best electric car battery on the market has terrible energy density.
(My gas tank holds 17 US liquid gallons, which is about 64.3 liters. I get a little over two 2GJ assuming 32MJ/L. This is about ten times the energy that the Tesla's battery pack holds, and weighs a tenth as much when full.)
The cost per megajoule of gasoline and of electricity at residential rates where I live are about the same. I went to my local electric cooperative's website and found that they're charging a residential rate of $0.0925/kWh, or a little over $0.025/MJ. By comparison, gasoline here currently costs about $2.70 per gallon, which is about $0.022/MJ--slightly less. However, the electric motors Tesla uses are, the company claims, something like 85-92 percent efficient, which is a lot more than any present-day gasoline engine* can manage.
So, as it stands, gasoline has vastly superior energy density, far more energy fits in your car at once, the cost per unit energy is currently about the same, and a gasoline-powered car doesn't lose any appreciable energy capacity over time unless somebody poked a hole in the tank, whereas batteries have to be replaced (at least $20,000 for a new Tesla battery pack, assuming that Tesla has worked out some awesome deal where they get lithium-ion cells at less than wholesale price) every few years. The battery is the biggest impediment to electric cars, and always has been. Electric car designs date back to the 19th century, and in that entire history, Tesla is probably the closest thing to a commercially successful product--and it's still lost over $200 million. The saving grace here is the efficiency of the electric motors (whose design Tesla has catered to the purpose). Then again, some of that gain will probably go away by having 450kg of battery sitting in your car. I guess there's also the additional incentive of feeling "environmentally friendly", which appears to be something for which people will pay a lot of money* (when they are not thinking about the disposal process of a 450kg lithium-ion battery that has to be replaced every few years, anyway.)
*Especially since the emissions regulations craze of the 1980s. Before that, it was not uncommon to see cheap consumer cars being sold that beat forty miles per gallon. Mind you, these were still somewhere well south of a third Tesla's claimed efficiency.
**Look at how much a Prius costs. If you want low TCO, buy a Jeep or something.