This is a discussion on Why Buy Apple TV 2? within the Apple TV forums, part of the Internet TV Devices and Peripherals category...
The answer is pretty much AirPlay which allows you to wirelessly stream and watch photos from your iPad/iPhone to your TV.
Otherwise if you're not interested in watching stuff from your iPad/iPhone on your TV and you don't care about iTunes content then you want a roku.
Oh, and there's also YouTube. I don't think Roku has an official YouTube channel just yet.