Simplified contacts manager
Address Book is another OS X app that always felt awkwardly basic. In OS X Mountain Lion, the rechristened Contacts is less embarrassing. The interface is simplified, with a pane that you open up to work on groups rather than go through that weird modal switch used in previous versions.
Contacts is also better integrated with other services. It gains privacy management capabilities and integration with both Facebook and Twitter, as previously noted, as well as with Apple's Messages chat app.