I was loving the look of the M3 - enjoyed the livestream - watched the first drive videos (http://bgr.com/2016/04/01/tesla-model-3-first-ride-videos/ )
right up until I got to timestamp 3:29 in the video, and they opened the trunk ... I can't put my dogs in the trunk if it's not a hatch! (and I'm certainly not putting them in the frunk). More to the point, lots of folks will want a more accessible loading area.
... I'm sorry, but for my money an affordable practical family car has to be a hatchback. The all-in-one glass roof is a lovely idea, but it's not compatible with a hatchback. A split roof, front 2/3 one piece of glass, back 1/3 hatch would be preferable. The all-in-one moonroof should go back on the drawing board until it's time to launch the Roadster2 (it would be amazing in a 2 seater sports EV).
I would suggest that Tesla urgently do some research on whether the 2/3: 1/3 split would be preferred by some of those 180,000 folks who have preordered ... I suspect they'll find most people will prefer to have a proper hatch, so the all-in-one roof will end up as the less used option.
There's a danger here that having promised not to overdesign, the allure of building the M3 with a single sheet glass roof will prove irrestistible, and this design own goal won't be fixed. The M3 will end up an impractical vehicle, and those who can't afford a MS or MX will sit it out in the hope Model Y solves the problem ... which isn't something Tesla can afford. Need to shift M3 by the shedload, and without a proper hatch ... folks will buy a Chevy Volt instead.