Feature	Summary
Shared Photo Streams	Accept invitations for Shared Photo Streams, browse photos and comments, and receive notifications of new content.
AirPlay	Send audio content from Apple TV to AirPlay-enabled speakers and devices (including AirPort Express and other Apple TVs). Also includes the ability to require an onscreen code to use AirPlay with your Apple TV.
iTunes account switching	Save multiple iTunes accounts and switch quickly between them.
Trailers	Search movie trailers. In the United States, see show times for local theaters.
Screen savers	New Cascade, Shrinking Tiles, and Sliding Panels screen savers.
Main menu	Reorder icons on the second page by holding down the select button on the remote.
Subtitles	SDH support for the deaf and hard-of-hearing as well as improvements to viewing and selecting subtitles.
Network configuration	Support for setting up advanced network options using configuration profiles. See http://support.apple.com/kb/HT5437 for more information.
Stability and performance	Includes general performance and stability improvements.

Here's the specific apple doc with a list of Security fixes in Apple TV 5.1. (Several other related docs in today's listing below.)
[FYI: Check the Apple TV forums, for some posts on problems with this update.]

  

• Mac notebooks: Operating temperature
• Mac notebooks: Spotlight indexing may make the fans run
• iMovie: Using Flip Mino and Flip Ultra camcorders
• OS X: About FileVault 2 (Lion/Mountain Lion)
• OS X v10.6: iOS device not recognized in iTunes after restart (w/tips)
• EFI and SMC firmware updates for Intel-based Macs (added Sept. 19 updates)
  
  
• Using AirPlay
• Apple TV (2nd and 3rd gen): Understanding AirPlay settings
• Apple TV: How to install a configuration profile
• Apple TV: How to configure a proxy using profile
• Apple TV: How to configure 802.1X using a profile
• About the security content of Apple TV 5.1
  
  
• About Apple Remote Desktop 3.6.1 Client
• About the (MLion) OS X Server v2.1 Update
  
  
• iPhoto and Aperture: Using Photo Stream
• Logic Pro, Logic Express, MainStage: Disable or remove Audio Unit plug-ins to troubleshoot issues
  
  
• iCloud: Backup and restore overview
• iCloud: About your icloud.com, me.com and mac.com email addresses
• iCloud: Photo Stream, Windows Vista, and iCloud Control Panel for Windows 2.0
  (iCloud v2.0 Control Panel for Windows D/L in Sept 19th news)
• Using your Apple ID for Apple services
• How to unsubscribe from Apple email subscriptions
  
  
• Cards: Frequently asked questions
• iOS: Safari web settings
• iOS: Importing personal photos and videos from iOS devices to your computer
  
  
• iTunes Store: How to use your iTunes gift certificate
• iTunes Store: Downloading podcasts using iPhone, iPad, or iPod touch
  
  
• iPhone: Hardware troubleshooting
• iPhone: Can't hear through the receiver or speakers
• iPhone: Hearing Aid Compatibility (HAC)
• iPhone and iPod touch: Tips for using Voice Control
• iPhone and iPod touch: Charging the battery
  
  (Updates from Saturday)
• iCloud: Troubleshooting iCloud Contacts
• iOS: Troubleshooting Siri
• iOS: Understanding cellular data networks

• Airfoil 4.7.3 (w/fixes for 10.8.2, iOS 6, more)
• Apple Remote Desktop 3.6.1 Client (for OS X 10.7 and later. included w/10.8.2)
• Apple Remote Desktop 3.6.1 Admin
  (On Aug 20th (before 10.8.2/10.7.5 release), Apple posted ARD Admin 3.6.1.)
• iPhone Configuration Utility 3.6 for Windows (Apple)
• AMD HD 8000 specs leak, point to major performance boost
• Samsung SSD 840 Pro (256GB) Review
• (Kim Komando) PCs vs Macs: How they stack up
• Rumor sparks new Facebook privacy concerns (again)
• Some iPhone 5 owners report scuffs/scratches out of the box
• iPhone iOS 6 Passbook "Can't connect to iTunes" (sponsor blog)
• How a Tiny Group of Designers Built the Most Efficient Racecar in History
• Sony STR-DN1030 AV Receiver Review