The appearance of Mountain Lion is essentially the same as Lion and Snow Leopard. It still uses the Aqua user interface, as well as the steel window color. There is very little difference.
iCloud -- Mountain Lion plans to fully embrace the abilities of iCloud. It will be using it for iMessages, Reminders, Game Center, Mail, Notes, and many other features.
iMessage -- Mountain Lion will bring the favorite feature from the iPhone, iPad, and iPod Touch to Mac. Messages. You will be able to send and receive messages for free from your Mac. It can be used exactly like iOS.
Reminders -- Mountain Lion will feature Reminders. It will pop up reminders at you from both your Mac and other iDevices. It will use iCloud to sync reminders to all your devices.
Notes -- This one is pretty clear. Notes for Mac. It will use iCloud to sync your notes to all your devices. You can also create a note and cut it out and place it on your desktop just like Sticky Notes from Windows 7.
Notification Center -- OS X Mountain Lion will also have the Notification Center. It will have notifications about your Mac and Mac Apps, along with your other iDevices that have iCloud set up. It will even have a Gesture that will pull up the Notification Center just like on iOS 5, which is done by swiping two fingers to the left.
Share Sheets -- Now OS X will have the useful share Sheets. Just look for the sharing icon on some apps to show a send to... just like on iOS.
Twitter -- OS X 10.8 will also have enhanced Twitter capability. You will be able to tweet right from your apps. Also when you get a mention or a direct message, a Twitter notification appears on your desktop.
Game Center -- One of the most important add-ons is Game Center. With Game Center you will be able to see what your friends are doing, check your stats, and more just like you have iOS.
AirPlay Mirroring -- Now, just like on iOS you will be able to wirelessly mirror your Mac to your Apple TV.
GateKeeper-- Gatekeeper in OS X Mountain Lion makes the Mac safer than ever. It helps prevent you from unknowingly downloading and installing malicious software. And it gives you control over which applications to download and run on your Mac. Now you can choose from three security options. You can download and run applications from anywhere, just as in OS X Lion. To be even safer, download and run apps from the Mac App Store and apps with a Developer ID. Or download and run only apps from the Mac App Store — the safest setting of all. Gatekeeper lets you decide which setting is best for you.
More Chinese features – has additional features for users in China, including support for Baidu as an option for Safari search engine, QQ, 163.com and 126.com services for Mail, Contacts and Calendar, Youku, Tudou and Sina Weibo are integrated into share sheets.
Time Machine is able to do rotating backups on more than one storage medium.
iCal is renamed "Calendar"
Address Book is renamed "Contacts"
iChat has been enhanced and renamed "Messages" (see above)
RSS support in Mail and Safari has been removed
Software Update – has been unified into the Mac App Store
X11.app – users are directed to the open source XQuartz project instead