I've been using C3 for a few days and I'm consistently impressed with how rich and full-featured it is. Scirra is hitting it out of the park IMO and it's a super solid base for the inevitable stable release.
That said, my complaints are mostly cosmetic (it looks... quite ugly, haha.) My other three nitpicks so far:
1. The tab bar wraps and it looks hideous... TBH I'd suggest Menu/Save/etc. and Username/Avatar on one line, and the tabs on the next, and then have all the other panels below them. Horizontal space for tabs is at a premium and if the properties and project panels, for instance, have to scroll an extra 30px or whatever I'd prefer that.
2. There's inconstancies with dialogs, where OK/Cancel are reversed in some cases.
3. Sometimes the fade/show animation on menus, like the user one, feel really slow. Make 'em 0.1ms.