My first thought is how to unshare a folder and thank God, it's already been there: https://www.dropbox.com/share#tab:linked-items

So, if you feel like "I wanna share this folder to the wild world, now!", just do it! You can change your mind at anytime later and the folder will be no longer accessible. Very intensive, careful design & implementation!

I think you're overstating the risks of random URLs. Unlisted cell phone numbers are not considered public even though you give them to all your friends and to every nearby cell tower. Credit cards are not considered public even though you might hand yours to a lot of strangers.

Still a good analysis of using the security by obscurity model for URL's

By default all my folders are linkable.

Can you tell us how to change that?