I don't know whether Motorola or Google is responsible for the next bit, but somebody decided the phone should stop charging when its temperature exceeds some maximum. That's clearly better than letting the phone ignite or fail, but my beef is that the phone doesn't warn me when it happens.
I've solved the problem by getting my a/c fixed. I mount the dock on the corner of the windshield, which holds the phone right over the little side vent in my car's dashboard. The stream of cool air emerging from that vent keeps the phone from overheating.
One interesting change in the D2 is that you can plug it in to charge w/out turning it on...the D1 always booted up when plugged in, on the D2 you get a large battery/charging icon and the phone won't turn on unless you turn it on yourself.
I also have my D2 mounted in front of an AC vent (using vent mount) for the same reason noted above....when using Nav + Pandora + maybe my wife is browsing web pages, having the phone sitting up in a windsheild mount in the sun as well is a bad combo - in front of the vent it stays nice and cool...