Potential Federation members should have developed warp drive, making them warp capable, and they should also have achieved planetary peace. I cannot think of any more criteria for joining the homosapiens only club?
However, a Star Trek episode which deals with a Federation member being disbanded has *never* been dealt with before, as far as I can recall, and I think that this would be a *really* good storyline. Especially if the species was an established canon Federation member, not a random alien species of the week.