Most of the truly interesting and unusual stuff (above and beyond the fact that it's really great for playing music and organizing your music collection) requires ethernet, and only the Mk2 has ethernet.

And I know what your next question is going to be (can I plug in a USB-ethernet adapter), and that answer is no.

There was talk of making the MK1's USB port emulate one of those USB-ethernet adapters (kind of the reverse of plugging a USB-ethernet adapter into the player), but no one has yet tackled this one.

In the meantime, the MK1 is a fantastic music player.