As suggested, the biggest sell is getting iTunes store content onto your TV, not a computer.
In general, some of the strengths vs. similar competing electronics are:
1. Simple, Apple-esque interface designed around the use of a remote control instead of computer and mouse.
2. Ability to stream purchased content from your computer's iTunes library to your TV (then take it with you on your iPhone when you're on the go).
3. If you don't watch a lot of TV shows, the new Apple TV may let you eliminate your cable bill while watching the same content for a fraction of the price. Since only two networks currently support the "rental" model, however, you may have to BUY most content through iTunes on your computer, still.
3. Portals to YouTube, Flickr (w/slide shows), and other internet services.
4. Free, on-demand content from video podcasts (I watch full length MSNBC podcasts).