We're limited by the third party wrappers we use (CocoonJS & directCanvas). Neither currently supports Game Center, although I think Ludei have plans for it. But the ball's in their court. The same goes for ads and IAP. We have implemented CocoonJS's IAP functions, but I'm not sure they've totally finished it, because it's not clear from their site how to use it. @Ludei - can you comment? Other features like the Facebook object are tricky to implement because they use a different API to the one used in browsers, which means we have to rewrite the whole object.
Where we have the ability to develop solutions entirely ourselves, we've got better options - like on Windows 8, which integrates IAP pretty well AFAIK. So we're aware it's important, but it's not always possible for us to solve it completely by ourselves.