Share Sheets. Many of the applications in Mountain Lion will include Share Sheets, the menu of services that pops up when you hit a share button. Sharing in Mountain Lion is just like using the Share button in iOS. For instance, someone in Safari can hit the Share button to quickly email a link to a friend, bookmark the page, add to their reading list, send it in a Message, or post it to Twitter.
Twitter. Twitter is now directly integrated. This makes tweeting possible after logging in only once, through the operating system. Users can use the share button in their applications to tweet links and images. When users receive an @Tweet, they will be notified in OS X, without having to use a desktop Twitter client.
Game Center. Now Mac users can play games with iPad, iPhone, or iPod Touch users thanks to Game Center. You create a Game Center account with your Apple ID. After that, it's just a matter of signing in and competing with players on Macs and iOS devices. Users can play with friends or meet random players or check worldwide leader boards to see who the top players are.
AirPlay Mirroring. Borrowed from the iPad, AirPlay Mirroring displays everything that's on your Mac on your television screen via Apple TV. You can use mirroring to watch videos and view websites on your HDTV. It's pretty cool for using a larger screen while kicking back on the couch.
China services. It's nothing most of us will be using, but if you use QQ, 163, or 126, Mountain Lion's China-specific features might be of use to you. It also adds a built-in Baidu search to Safari.
The Share Sheets add Youku and Tudou video sites as well as Sina weibo, a microblogging service similar to Twitter in China.
Mac OS X Mountain Lion debuts this summer. It's likely to be a Mac App Store download, and we're willing to bet that it will run $29.99.