FTA, it looks like WP7 is trying to walk the fine line between Apple (little-no customization) and Google (full on open source) in its user experience.

If they're able to continue staying on this line for an extended period of time (say, for two or three generations of the OS) they can be successful. However if they start falling on to one side (which they have in the past), they will be seen as a me too in the space and their brand will suffer.