In Pictures: OS X Mountain Lion arrives - the top ...
Automatic syncing of accounts and mail rules
I hate setting up Mail rules on multiple Macs -- there's no way to export or transfer them, so you have to reconstruct your rules on each Mac and keep them all updated. No longer: OS X Mountain Lion automatically syncs your Mail rules and other account settings across all Macs using the same iCloud ID (and running Mountain Lion). There's nothing to configure -- it just works. iOS 6 will support account syncing with both iOS devices and Macs as well.
In Pictures: OS X Mountain Lion arrives - the top 25 features