One thing we know as of right now with Windows Phone 7 is that customization will be drastically limited. At least until the folks at XDA figure out to root and mess with the OS.
An example of what customizations you can do will be the use of "themes" which is a generous description for changing some font colors and accents. Yeah, basically it's not much. One example is in "Pivot" (below) the other "Panorama" (above) which are two of the formating options for programs by developers.
Still, it's better you know now what you can and can't out of the gate then later. So commence logging your complaints in comments...now.