By which I mean: they don't understand basic principles of operating a motor vehicle.
This has been a painful realization for me. For most of my life, most bad drivers were people who either knew the law but chose to drive otherwise, or who simply didn't know the law. It was only a tiny subset of drivers who showed no indication of, for example, the size of their vehicle, or how long it would take to bring it to a stop, or how to park. (Not parallel park, just regular park. Like the kind of parking where you just point the front of your car at the place you want to park it, and then put it in that place. I see someone stymied by this intractable puzzle almost daily.)
The last couple of years I've noticed a rash of people who not only don't know the law, they show no clue of knowing how to drive. They don't know how wide their cars are, whether their turn signals are on, how fast they're going, whether the stoplight is green, or other basic driving facts.
I don't think we need to outlaw texting, or improve road safety with hinky methods with roundabouts or somesuch. I just think that getting a driver's license should be at least as hard as passing seventh grade.