While the Apple TV's form factor remains the same, the software interface has changed significantly. The most notable feature is the ability to buy music, movies and iTunes rentals directly from the device, no computer required. Frankly, this is something I think should have been included from the very beginning. The Apple TV will be able to playback HD and standard definition movies and support Dolby Digital 5.1 sound.
Additionally, Apple TV will now support interactive slideshows via Flickr and .Mac accounts.
There's one more change - one I also think is a big improvement to the overall product - the price has dropped. The 40 GB entry model is now just $229 USD (down from $299) and the 160 GB model is $329. Now that users actually have the ability to buy content directly from the device, and that HD content will be readily available, it has suddenly become much more attractive.
Current Apple TV owners can get the additional benefits via a software update