Good means a good reputation and respectable work in there, since a site with a bad rep or mere snapshots probably won't get too many visits by interested people, and will lessen your chances of selling anything.
Good in terms of photo qualities. I think istockphoto does a good job in telling one what are good submissions and what are not. They moderate the submitted photos too; something, in my opinion, which is good; it helps me improve on my skills.