Is there a way to make notes, or possibly change the titles of posts that I save in my bookmarks - so I can see at a glance why I saved them in the first place?
Not with the way bookmarks are currently implemented here, no. It just saves the ID of the page you are saving. More full featured bookmarks would be cool though.
I always used Delicious for that sort of thing but that site is EOL. Not a fan of Pinterest.