What I've found...
-Photo thumbnails (scroll through them. click to view it big, and click play to start a slideshow from that photo)
-Streaming of photos from mobileme (obviously) and from iPhoto on your mac.
-Trailers view has changed to big thumbnails of movie posters, when clicked on one it shows the movie's info. very similar to the rent/purchase screen on the appletv store
-The 'Clear' button has been added to the onscreen keyboard. Accompanies 'Delete' and 'Space'
-When streaming videos (I've only tested streaming from my iMac), you no longer see the buffering/loading bar thingy on the screen. It just starts playing.
-iPhones and iPod touch's can now act as remotes (obviously). In Settings>General>Remotes it has an option 'Add Remote' (currently dimmed out for me). When you hover over that option the 'Remote' App appears on the left of the screen with this description....
"Control music and video on Apple TV from anywhere in the house with an iPhone or iPod touch. Start by downloading the free Remote application from the App Store."