Vote on features that you would like to see implemented in Media Center: http://mediacenterguides.com/feature_requests.
This is by no means an all-inclusive list, but it does have many of the most requested features for Media Center. Feel free to discuss the details of each feature using the comments on the site, or by replying here. You can also see the (nearly) all-inclusive list here: http://thegreenbutton.com/forums/thread/191773.aspx. Sorry for sending people to another site, I just didn't want to re-post everything, and I thought people here would still appreciate the information.