If you provide a check-list or other criteria, be prepared for a world of pain. There will be people who claim they qualify, and they won't let go. They really, really won't let go.<p>They will harrass and harrangue you, they will call you liar, cheats, thieves and worse in public, and they will question your parentage and, what's worse, your ethics and morality.<p>And they won't stop.<p>I speak from experience.<p>You may choose to publish somewhere a list of things you consider, but say it's incomplete and for guidance purposes only. You may choose to provide a form response to those who you choose not to accept/list/whatever, and you may say that while it's no doubt worthy in it's own right you've decided it's not a good fit for your service.<p>Just don't be specific.