I'm also missing very basic html/CSS functionality. Especially something like divs and tables or equivalent. Maybe it could be used for ingame UI and such as well.
CSS transitions is one of the most missed features. The card flip feature in this thread is pretty neat.
I played around with it a litte bit. It renders a bit slower than sprites. If I'm not mistaking you also cant have any of these objects inline in between layers. They are always on top, rendered separately from canvas.
I would love to have CSS styling and transitions on sprites, or at least some equivalent, but I don't know if it's possible or not.
I friend of mine made this: http://chimely.co/ and I would love to make similar apps/webpages but using C2, as it's such a breeze to work with.