Also a non-caste based system would seem to be required, as having a caste based sytem would have excluded Bajor from entry.
I assume a cast system is one of many social problems that would exclude a planet from joining.
I doubt the Federation would have allowed the Cardassian Union to join the Federation, when they were still under a military dictatorship. Cardassian society doesn't have a caste system persay, but they do have a oppressive government that combines the worst aspects of Hitler's and Stalin's regime, that would seem to conflict with the Federation's values. In Drumhead, it seemed like were was some sort of bill or charter of rights for Federation citizens, so I doubt they let in societies who did have similar values.
Also in Hunted it seemed unlikely that the Federation would let the Angosians join, until they resolved the situation with their veterans.