I'd actually love a write-only folder. Then I could give it out to everyone who, say, attended a birthday party, and say "everyone drop off your pictures of the party in the folder" without worrying about one person clobbering the whole folder.
Register with the host machine a protocol handler. That way, I can email my friends btsync://ABCDEFG and if they have the app installed, clicking on the link will bring them directly to the "Add Folder" window with the shared key pre-filled and read-only. Ideal when dealing with less technical folks, otherwise, you have to explain in the email the whole "Go to the folder tab, find the little + button, click it, copy and paste in the key, select a folder, etc etc"