Excellent idea Rated 5 out of 5 stars
I read about this at http://www.pcworld.com/article/172854/notawesome_firefox_addon_selectively_hides_your_history.html and had to check it out. I'm giving it 5 stars not just because the name, NotAwesome, is awesome, but because it's a great idea, very handy and super-easy to use.
That being said, for people unfamiliar with managing bookmarks, a little tutorial or explanation might be helpful. I was quickly able to figure out that it's managing the hidden bookmarks with a "notawesome" tag, which you can use to find your hidden bookmarks or remove to unhide bookmarks, but perhaps some people might have difficulty figuring that out. Pointing out the "Recent Tags" line in the Bookmarks dropdown would also probably be helpful. In fact, it might be best to add a "notawesome" option to the Bookmarks dropdown (Yoono is one add-on that does this) and then give people options for viewing and managing their hidden bookmarks.
It's still experimental, but it's really good and I'm sure it'll become even better.
+2 for mongoes idea.