Odd, when I learned to drive I found this immediately obvious the first time I actually turned using a signal...
The indicator is on the left side, so if I'm making a left hand turn then my hand is moving down--the same way that the signal moves to signal left. In fact, if you are holding the wheel in the right place and extend your fingers while you are turning then your hand clicks the indicator correctly without you having to think at all. I think that, aside from the steering wheel, it's one of the most intuitive things on the car.
I agree with you about the direct connection between the user's movements and the speed, but the direction of the twist required to go forward/backward is fairly arbitrary (it seems), so it must be learned rather than 'intuited'.
As for the throttle, you're quite right - the twist direction to go forwards is arbitrary, but the rotating all the way in the opposite direction to reverse (which is what I wanted to highlight) makes much more intuitive sense to me than throttling down to zero, moving a lever, and then throttling back up again.
I found the system to be rather touchy. When coming to a stop it was quite easy to overcompensate and start oscillating back and forth. It was quite fun to drive though, and for the kind of work a bin carrier does the ability to reverse without changing gears is very useful.
Incidentally, in order to achieve this continuously adjustable drive a diesel engine runs at a constant RPM turning a hydraulic pump which in turn drives hub motors in each wheel. The whole thing makes a very distinctive sound as it drives around.
With those engines, there's a very direct connection between the user's movements (moving the tiller) and the way the boat moves in response. It's not immediately obvious, but it becomes natural once you create a mental model around it - in the same way that using a turn signal on a car isn't immediately obvious (why does moving a swingarm down equate with going left?).
The real part I like, however, comes with reversing. As opposed to having to explicitly shift gears, simply turn the throttle in the opposite direction as you do to accelerate. Counter-clockwise to accelerate, clockwise to slow down, and keep turning it clockwise (through a safety stop) and you'll end up going backwards, which you can consider the extreme manifestation of slowing down (you have slowed so much you have negative forward velocity).