I'm looking forward to seeing how these devices play in real world usage. The two things that jump out at me immediately.
1. Most important: On a device with a small screen pixels should be precious. It worries me that many of the built in apps use a significant and unchanging amount of the screen to tell you what app you are in. I don't know if it will really impact usability, maybe there is enough screen real estate to go around, but it seems wasteful. Perhaps the fact that many of the different app's UIs (especially the list UI and the list of tiles UI) look very very similar makes it necessary for you to have a 1/5 screen reminder that you are in the photos app and not the marketplace?
2. The multiple message selection is the best example of this, but there are others: many of the possible interactions with on screen elements are completely hidden and non-obvious. It would be difficult to discover that you can manage multiple messages by tapping on the far left hand side of a message. Contrast this with the iPhone, where you tap edit or whatever, and the message management UI comes on screen. It reminds me a little of the Anti-Mac interface (http://www.useit.com/papers/anti-mac.html) in that it is actually faster (you don't have to go and hit a button that takes up space on the screen in order to start moving messages around) but at the price of discoverability. Are there other examples of Windows Phone 7 being designed for expert/informed users?