After the windows mobile session at MIX09 these are the key things that I gathered.

1. No more honeycomb - users didn't like it. The Start menu icons will still be aligned in a staggered grid, but they won't have the border.
2. The Zune-like Today screen can be navigated by sliding the selector...thus moving it up and down, or by sliding the background, thus keeping the selector stationary.
3. According to the presentation, the Marketplace will be launched in September or October.
4. Like the app store on the iPhone, the Marketplace will allow users to see apps by Featured, New, Most Popular, and Category (of which there are 13). Also, there will be a robust search.
5. Like the app store on the iPhone, updates are automatically issued for apps.
6. Like the app store on the iPhone (are you noticing a trend here?), users can rate and review all of the available applications.
7. Developers will have access to all of their uploaded apps through the "Developer Dashboard." Here they can see graphs of sales trends, plus they can add new apps with screenshots/descriptions/meta data.
8. Applications are subject to screening against 60 pieces of objective criteria to make sure that the submitted apps are not malware, and to ensure compatibility.
So, it looks like Microsoft is taking the Marketplace very seriously be insuring that the user gets a fantastic experience both with the Market place and the app itself. We're dying to see what this thing actually looks like (and at this point, it's not likely to be just a mobile-formatted website). Hopefully we'll get a glimpse of the UI soon!

P.S. A lot of the big name developers for Windows Mobile software are already getting access to the tools and (presumably) the Marketplace app itself. They are under NDA - Microsoft is trying to keep the Marketplace tightly clamped down until it's fully baked.