hmm... good question.
Well, the ability to search my old posts. That would convince me. Though I think that should just be possible without paying. But I would pay to be able to do this.
How about a guaranteed, saved slot to mint mints? Just one. So you don't miss stealth mints. Obviously people could pay on multiple accounts, but that just helps you out. If that is problematic - only accept one payment for green per card if possible. That limits people to the amount of credit cards they own at least. (Disclaimer: I don't actually care about or need this because I am terminally online, just a good idea i think if you could limit it to one or a few per person).