Adding onto UI / UX, is the tabbed (pin button) ability gone? I don't see a way to tab windows to the side. Would like that feature to return.
Stylistically, I think the tabs in C3 are way out of place and are kinda ugly and don't fit in with the theme at all. They shouldn't be rounded. Actually, almost nothing should be rounded in this sort of minimalistic UI. The grey rounded borders on interfaces should probably be removed.
I'm not a fan of the new project and properties bar, it a has a lot more white space and ends up showing a lot less information in a lot more space. I have a lot of things going on and want to see as much data as possible without having to scroll around. A great solution to this would be a zoom in or zoom out.
Additionally, I think the text a lot less sharp than what we had in C2. It doesn't feel as clean. The text in event sheets and some of the other places is still good but I wish we had that same font in places like the project bar.
Kinda unrelated but I think Scirra should really consider using a service like https://www.uservoice.com/ to prioritize new features and bug fixes. Looking forward to seeing C3 grow.
One other thing, It was a good choice to italicize event sheets, but that needs to be consistent, why aren't they italicized in the tab headers?