Cheryl Simmons has posted a concise list of Windows Phone 7 Design Guidelines, which I am sure will be helpful to all Windows Phone Developers.
I am quoting some of the points from the original post:
Themes: Avoid using too much white in applications, such as white backgrounds, as this may have an impact on battery life for devices that have organic LED displays.
Application Settings: Application actions that overwrite or delete data, or are irreversible must have a “Cancel” button.
Canvas/Grid for layout: Canvas uses a pixel-based layout and can provide better layout performance than the grid control for deeply embedded or nested controls in for applications that do not change orientations.