I've mentioned this to friends in the past, mostly because there is almost never a good reason to drive on that part of Market St. Its two lanes in each direction, you can't turn left anywhere, and the interior lane is supposed to be for Muni only, and the right lane is often blocked by people attempting to turn right... who are held up by pedestrians. And then there are the delivery trucks/vans, and the cabs, which can block part of the right lane (or just pull up onto the sidewalk, even more fun).
I figure most of the cars on that section of Market are tourists or other people who just don't know any better, and then they're stuck on it, unable to get off.
Of course, closing it down presents problems, probably the largest is with deliveries to businesses along Market. I wonder if you could solve most of the problems just with signage, ie "No turns except Muni/Cabs/Deliveries" or somesuch. That would be helpful to those who don't know any better, though they'll probably just go WTF? It wouldn't allow you to transform the street, however... imagine turning it into just a single lane in each direction for Muni, for example.
Anyways, here's hoping that whatever comes out of this isn't a really bad idea... I'm guessing there's an 80% chance that nothing comes of this, and a 20% chance that something bad comes of this, that's about how politics in SF goes...