I like one decision they've made. The icons are big (Microsoft calls them "tiles") and they aren't just convenient points to press to launch an application. They are useful in-and-of-themselves. I only use a few apps over and over, so I don't need to see 20 icons at a time. (Your usage may vary.)
Here's Computerworld's Dan Rosenbaum very positive write-up of two early WP7 phones, the Samsung Focus and HTC Surround:
The first thing you'll see when you fire up a WP7 phone is an interface that will knock your socks off. It's immediately apparent that Microsoft achieved at least three design goals:
1. Forget that Windows Mobile ever existed. Start with a clean sheet of paper.
2. Make a phone that is at least as tied to the cloud with Microsoft tools as anything Google could ever do.
3. Build an interface that's impossible to look at without getting information.
It's that third point that makes WP7 truly different from other phones. Where other smartphones use small icons that, aside from status badges, are pretty much static, Microsoft's large icons, which it calls tiles, are either in motion or tell you something substantive. An iPhone screen displays a 4-by-5 grid of 57-by-57-pixel icons, some with badges, all with captions. In contrast, WP7 tiles come in two sizes. The smaller is a roughly 3/4-in. square -- only two will fit across a phone's screen. The larger icon is the same height and roughly twice as wide, nearly filling the width of the screen.
Read the full story at Computerworld.