> - You can't share an arbitrary folder with someone, only a top-level one

You can share any folder arbitrarily deep. What we do not allow is sharing subfolders of already-shared folders.

> You can't share app folders

We plan to remove this restriction in the coming months.

> App folders are buried in the Apps folder

We are exploring how to better handle App folders, but the sharing API will not address this.

> This would let me share with both apps and other users.

Great point. Note that apps with full Dropbox or file type access already can share data with each other, just not two apps with only App Folder permission.

Are there specific (App Folder) apps you wish to share content between?