I'd really like to see a customizable threshhold on what is and isn't spam. I've noticed a lot of mailing lists and things I did sign up for getting passed to junk, though periodically I try to mark them as "not spam" presumably enough other people mark similar messages as spam that my suggestions are overridden when new messages arrive. Something as simple as a slider to control how zealously the filter culls messages from the inbox might go a long way to making users feel more in control of the situation (whether such a control could be effective enough not to annoy users and have a real effect on the false positive rate is another question, so I admit there's no easy answer).