There are two examples of what I am talking about right there in the image you linked to.

Firstly, what screen am I on ? I can't tell because at the top of the screen all I can see is the bottom half of three letters. Very pretty and designery, but not very discoverable.

The second example is right below it, I can see the first three items of a menu, "Artists", "Playlist" and "So..." But I can't see what the third menu item is (or if the menu extends beyond that) without scrolling, again not very discoverable.

Another example of this sort of undiscoverability I saw in the demos was the contact lists. There is no A-Z list on screen, you have to know to tap on the letter that you can see in the list to pop up the A-Z list.

It is the discoverability of the iPhone UI that makes it usable by "normal" people like my wife. I can't see the Zune-like UI being frictionless in the same way because of these discoverability issues.

But I'm being a bit unfair, given that I've never actually played with a Zune let alone a Windows Phone Series 7 Phone (snappy).