BuddyPress.org

No, this hasn’t been tried yet, because there is no need and it’s still in it’s baby shoes.

Limitations would be no cross-browser support.

No oppurtunities, unless offcourse IE suddenly desides to fully support HTML5.

HTML5 is made to make markup more easy and more readable, therefore your actually use tags that fits it’s content. For example <header> for the header.