This is what always bothered me about some parts of the US (eg California). Driving constantly in the left-most lane (instead of using lane for strictly passing other vehicles). If you drove that way in Europe, you would have a lot of angry drivers right on your rear bumper. Auto-Pilot will have an interesting impact on such behaviour and may make it worse.
It would be very courteous if everybody used the left-most lane for passing only (assuming the road-way isn't full of traffic). It could reduce congestion and allow faster vehicles to more easily get past.