Among other, it introduces a more robust security model where you can access databases without using SQL Server Logins. You just need to use the Contained User and you can have full access to the database.
This allows you to easily migrate Contained Databases to other instances of SQL Server 2012 without having to worry about any SQL Server logins, orphaned users, etc., for the simple reason that SQL Server Logins are not necessary when using Contained Databases!
I have just prepared a screencast with live demos showing how you can migrate an existing database to a Contained Database and thus achieve portability.
I'm making the screencast available here as well as on "The SQL Server and .NET TV":
Also, below you can access the slides.
Wishing you Happy Holidays and a Happy New Year!!!