Well, Apple TV broadcasts in 720p, while the iPod dock is limited to 480p. So if your source file is in HD you will see it in HD. That's one big difference for owners of big screen TVs.
The other difference is that the Apple TV is not just limited to the size of it's drive, but also to the size of your share folders, which presumably are larger than the size of your iPod storage drive. Presumably your Mac hard drive that contains the shared folders is larger than your iPod and can store your entire music/video/photo collection rather than just a portion of it. My collection of music/movies/photos is much larger (~1.2TB) than any iPod on the market, so I see the value there.
That being said, I don't think Apple TV justifies the price, especially if you're a gamer. The PS3 and Xbox 360 do the same thing, at higher quality (1080p) with added features for about the same price or just a fraction more.