IMO, those are the two most important issues when working with MFD.for me the issue has always been about focusing accuracy.
the autofocus on the mamiya has never made me feel secure
I have the magnifier and even that never made me feel very confident.
I was shooting with a hassy 110mm f/2 wide open and a mamiya 80mm f/1.9 and in the end had to rely on constantly checking the laptop at 100% to ensure my focus was right.
you can have the BEST lenses in the world but all that is for naught if the camera body does not give you the confidence of accurate focus.
Also, shutter lag is a horribly important issue, if you shoot people then even a fraction of a second makes a difference in the expression you capture.
I'm sure the phase one people dealt with it...I hope.
Focus accuracy should be a more important priority than the constant struggle to pack more pixels into a given space for all of the manufacturers. It will be interesting to see how well the S2 does in this regard ... being a totally integrated system, it has the potential to be very good, and could well make up for less pixels if the lenses are as good as advertised and the focus is dead on the money.
-Marc