First things, first: You have to get the Apple TV Software Update 5.1 onto your device before you can experience its promised enhancements. You can download the update by going to Settings -> General -> Update Software on your Apple TV and choosing to download and install the update. The update takes a few minutes to download.
Install the update. and you'll see a splash screen that outlines the latest changes (and other previously offered updates including support for Hulu Plus). Click Continue to move to a familiar interface.
To use the Shared Photo Stream feature, select the Photos app (which will show a number badge if there have been any additions to streams you've subscribed to), and click Select. In the resulting screen you'll see not only your iCloud photo stream (if you've enabled the feature on another device) but also streams shared with you. Next to each entry is the number of images in the stream. To view a photo stream, select it and press Select on your Apple Remote. For shared photo streams, you can elect to see images in a slideshow, use the stream as your Apple TV screen saver or unsubscribe from the stream. Select Settings in the Photo Stream screen, and you can choose to turn off your photo stream, turn off shared photo streams, or sign out.
Also within Settings, you'll spy an improved AirPlay command. Select it, press Select, and you discover that you can now choose AirPlay compatible devices to play audio streaming through your Apple TV (including other Apple TVs and Macs). Unlike with iTunes, you can choose just a single AirPlay-compatible device--you can't choose multiple speakers. On this screen, you can also choose the option to require a code before AirPlay can be used with the Apple TV.
Choose iTunes Store in the Settings screen, and click Select; in the resulting screen, click on the Accounts command. In the screen that appears, you can not only sign out of your account, but add additional iTunes accounts--the iTunes account of a family member, for example. Once you've added that account, you can switch to it, thus having access to the content assigned to that person's Apple ID.
The update also offers three new screensavers--Cascade, Shrinking Tiles, and Sliding Panels. You access them through the Screen Saver command in the Settings screen.
Within the Trailers app, you now have the ability to search your local area for movie times. In the U.S., launch Trailers, navigate to the Showtimes command, enter your Zip code, click Submit, and local theaters appear. Select a theater to see what's playing. Choose a movie playing at the selected theater and you can see its showtimes. On this same screen you'll find other theaters in the area also showing the selected movie. Choose one of those theaters and you'll see its show times.
The update also includes the option to rearrange icons for third-party apps on the Apple TV's home screen. (You can't rearrange the Movies, TV Shows, Music, Computers, and Settings icons). For example, select Netflix and hold down the remote's Select button. The app's icon will begin wiggling. Use the direction buttons on the remote to move the icon to a new position. When you're happy with its placement, just click Select again to plant the icon in its current position.
Finally, the 5.1 update adds support for SDH (Subtitles for the Deaf & Hard of Hearing) subtitles. Apple also claims that the update includes general performance and stability improvements.