I think the two most important questions will be answered when reviews start coming out:http://www.mirrorlessrumors.com/canon-repeats-no-plan-to-make-any-pro-mirrorless-camera/
1) How quickly and accurately does it focus?
2) How good (optically) is the 22mm f/2?
But above those two is a question only Canon can answer:
1) Where are the other lenses for the system? / Where's the road map?
According to a few youtube videos, the AF is quite slow. IMO Canon just want to prevent current EOS users from buying other brands when mirrorless coming into their minds, because they can mount whatever canon lens onto this one. That's the only selling point, other than that the EOS M seems a generation behind others.