I would love a simple, personal notes taking feature. Not for everything on this page, but just on my personal page. Just for me to take some personal notes about stuff on this page, some useful metaqueries, links related to this site, etc.
I know I could use bookmarks, but those are only local to the browser, and I use the site from different places a lot, and so, for taking notes about a page, the page itself is the most logical location.
And it's only one extra column in the user table, so I shouldn't take too long for you to develop :))