They also offer a full mobile back-end (increasingly referred to as MBaaS...admittedly not the nicest acronym): http://en.wikipedia.org/wiki/Backend_as_a_service
One of the biggest limitations is the length for which cloud code methods can run. (Nothing longer than a second, IIRC)
You can build a social sharing app, or share some data between apps, but building a "product" on it is limited to very simple cases.