The "button" solution can be cracked by exactly the people you'd rather not send you any email, ever: people who have a lot of time on their hands and can spend a day pressing a button (or, worse, have someone in India pressing the button for them).

The cash solution is interesting, but again, do you want only rich people to send you email?

I think the prioritization problem is hard because it can only be done by the recipient (not her secretary, not her colleague, not anyone but her).

But I also think that people hate to prioritize not because it takes time, but because it's incredibly boring and then stays in your mind (you remember you were bored, and you remember all the useless crap you had to go through).

But very important and very busy CEOs still watch TV or go to baseball games, so they do have time left; it's just that they'd rather watch TV than sort through their mail.

So the problem becomes: how do we make prioritizing fun?

(I have no idea how to answer that question, but rephrasing the problem is a first step).