To prompt some discussion, say someone tried to build "self-driving" cars in the following way: put a big brain full of rat neurons in a vat, hook it up to a car, and then train it to navigate.

I don't think that would solve any of the problems of driverless cars. The problems of driverless cars are about handling a lot of edge cases that don't come up often that a human can solve by having a decent mental model of the situation he's facing.

Why should we expect those rat-brain to have good mental models?

According to Musk self-driving cars while need to have a 10x reduction in traffic accidents to be viable. That won't work if you basically train mouse brains to do the same thing that humans do.

Is the distinction between human neurons and other mammalian neurons significant?

Mice with human glial cells are smarter. New Scientist, Cell Stem Cell (pretty sure that's the paper, but it might be an earlier one)