I see all the complaints about this just being a tiny box with a sensor and a lens mount and little else, and you're right; but I think this is the POINT. It seems pretty clear that Sigma is thinking of this as a modular camera, with only the most essential parts built in so that the end users can set it up with the parts they need; so you're not paying for components you won't use (like a viewfinder or hotshoe if you're buying it to use as a video camera, or a big grip if you're going to put it on a tripod or into a steady cam rig, or microphones and sound ports if you're buying it to use as a stills camera, etc.)
This is what I was getting at with my comment above about this little box possibly representing the real flexibility that should be unlocked with modern digital camera design. If this thing offers truly excellent imaging from a top notch sensor and access to one fo the best and most versatile lens mounts out there, and everything else can be customized by the user for the exact purpose they intend, that could help keep waste, redundancy, and cost down. The argument that the box is too small to bother with if the lenses are huge is ignoring the modular intent. If you use this little box as the only part it is (sensor and lens mount) and put on the other parts you need for your purpose (as the promotional video clearly shows is their idea for this) then it's only as big as it needs to be; all the other parts get added to do what you need. Thinking about this primarily from the point of view of stills shooters, maybe this seems silly; but nobody building cameras these days (besides Leica...) thinks of these tools as only for stills. Video is, if not the primary purpose, at least needs to be a serious consideration. Have you folks looked at how video shooters set up their cameras these days? Especially pros are always using separate monitors, focusing rigs, microphones and other sound gear. From that context this design makes a lot of sense. That Sigma has also thought of using this is a stills camera, and provides add-ons for that is just icing to make this more appealing to those who might want to use a body like this for both stills and video.
Just my 2 cents. Anyway, as I said above, I look forward to more info. Once we see reviews, sample images, pricing and availability, battery life, etc. etc. etc. we can re-evaluate the purpose and the relative success of a this camera. But if they get it right, this could be a real winner for Sigma. Maybe it won't be for everyone, but if they implement this right, it could be an incredibly useful and versatile basis of a truly complete and wildly flexible shooting platform for lots of users.