Sounds minor, but I would have loved to have been able to change the SMS screen so I could have a black background instead of a white one. Is it that hard to do?
I set up the accessibility feature called Invert Colors and assign it to a triple tap on the home button. That works great for things like Mail and Messages, especially first thing in the morning when my eyes are tired and I don't want my retinas seared by a crisp white screen.