Privacy concerns aside, this is pretty neat. XAuth (bad name, has almost nothing to do with authentication) creates a level playing field for sites and helps to create a customized (and hopfully more meaningful) experience for users. At the most basic level tools like “share this” will now show sites you interact with no matter how large or small. It also should help to break down the walls between sites and chip away at “the big 4′s” dominance.