Picasa, on the other hand, is best piece of software I've seen in a long time, especially regarding these factors. Unfortunately it doesn't deal with mail.
Your mission, should you choose to accept it, is to build from the ground up an all-new innovative POP/IMAP mail client. And of course it should integrate with Gmail in some way, but I'm sure you can figure that out because you people rock.
I've never yet paid for a mail client application, because they all suck. But if you create the perfect one, to me it would be worth every penny of whatever you decide to charge for it.