One of the flagship iOS features to carry over to OS X Mountain Lion is AirPlay Mirroring, which allows users to mirror their desktop to an Apple TV. System audio can also be streamed to AirPlay-compatible devices, which means that apps without custom AirPlay support (like Spotify) can stream directly to an Apple TV or Airport Express.
AirPlay Mirroring is a great way to share content from your desktop, no matter what program you're running. It works well even for streaming video sites like Vimeo.
You can control AirPlay Mirroring via the new AirPlay menu bar icon, which shows up when there's a valid target device on the network. You get some additional display options once you've turned AirPlay Mirroring on:
When enabling AirPlay audio, go into Sound>Output in System Preferences and choose which compatible device you want the sound to go to:
In my testing the mirroring worked great, but some issues can arise when you turn AirPlay Mirroring off. I'm using a mid-2012 13-inch MacBook Pro and after AirPlay Mirroring was turned off, it still had my Mac's resolution adjusted to that of my TV's. To fix it, you'll have to open the display panel and select your previous resolution. However, after a few times using AirPlay Mirroring, the problem stopped. Richard Gaywood noticed similar issues in his review. Unlike Richard, my MacBook Pro wasn't running hot with AirPlay Mirroring going, but it also has a newer graphics card.
Other than this issue, I think that AirPlay Mirroring is going to be extremely useful. The requirements are pretty tight; if you have a computer that wasn't purchased in the past year or so, it probably is not supported. If you're looking to get AirPlay Mirroring features for older systems, AirParrot will fill that gap nicely.