The only difference between emails, blogs, wikis, and notebooks is permissions.
See the table below, which lists the document type, view permissions, and add permissions.
Other than that, they can use exactly the same technology (RSS / Atom), and exactly the same authoring WYSIWYG methods.
Currently, they're all separate tools - I can't see that continuing. We're likely to see integrated online authoring suites to handle all of them together.