Currently when you view your user profile and select your reputation history, you get a lovely little chart detailing the growth of your reputation score, and you can select a range of dates to see a list of exactly what questions garnered you how much reputation during that period. However, the range MUST start on a different date than it ends upon.
Being the sort of geek who loves to watch numbers go up, I would like the default graph to have the ability to display one single day's worth of reputation so I can see, for example, how close I am (or was) to hitting the rep cap that day, and which questions/answers did best on just that day.
If this does get implemented, a nice shortcut might be to have the dots on the chart be double-clickable to display that one day.