You have to realise that there is no operating system as such on the xbox. Everything needs to be coded from scratch.
So even though the xbox is basically a pc anything that seems simple to do in windows still takes a lot of time and effort to code for the xbox.
So as lucas has said, in order to share a folder on the xbox you would need a full samba server integrated into whatever dash you were using. It's not too difficult to code a samba client (to access samba shares on a windows pc) but a samba server is a whole other ball game.