Bugs aside, I'm mostly liking C3 so far but the flat design makes things a lot harder for me to read. Support for custom skins or just a C2 style would be cake.
And an option to disable traditional math symbols appearing when not editing a field would be great. Looking at something and not seeing exactly what I typed is just confusing sometimes and only slows me down.
All my other UI issues come from the browser, so I won't list them since they'll be addressed by the .exe version.