How does this compare to http://owncloud.org/ ? I tried that out and was pretty impressed by what came out of the box and its integration with Gnome 3.
1/ cozy is a PaaS : plateform as a service : you develop in your language (node.js, ruby, pthon... only node for now) and your app is automatically deployed on the user's instance (if he ask to, of cours :-)
2/ data are shared among the apps : several apps can require access to your photo, contacts or mails or whatever : your apps are collaborating like on your smartphone.