The two main benefits of the Federation are
Starfleet is a formidable force, with most of its starships capable of posing a threat to almost anything in the Alpha quadrant. In this way, Starfleet is more like NATO, where you retain sovereignty but are obliged to contribute towards your common defense. Attacking a Federation world obliges a response by Starfleet.
No society can join unless it is warp capable (the TNG episode First Contact involves contacting a society that is on the cusp of that breakthrough). Otherwise, the Prime Directive applies, where they cannot reveal themselves, or their technology.
We don't see much in the way of a full list, but we do know a few things
You need a unified society for the most part. Admissions of societies that were not unified were uncommon, and done under an "associate" membership (TNG Attached)
PICARD: Every member of the Federation entered as a unified world, and that unity said something about them. That they had resolved certain social and political differences and they were now ready to become part of a larger community.
Those divisions basically sink the application of the Kes
RIKER: That's it! I can see that diplomacy is not going to get us anywhere today, and I do not have time for to negotiate. So let's put all of our cards on the table. You're concerned the Kes are going to be admitted to the Federation.
RIKER: As First Officer of the Enterprise I think I can promise you that's not going to happen. The Kes will be denied membership.
MAURIC: You have no authority to make that decision. Despite whatever games you played with the Prytt when you arrived, we still plan to take our petition directly to the Federation Council. They'll listen
RIKER: They will also listen to the reports of the Captain of the Enterprise and his First Officer. And I can tell you right now the First Officer's report will go something like this. Kesprytt, a deeply troubled world with social, political, and military problems they have yet to resolve. The Kes, while a friendly and democratic people, are driven by suspicion, deviousness, and paranoia. It is the opinion of this officer they are not ready for membership.
The Federation also has certain moral standards. Among them are prohibitions against slavery (Picard isn't happy with the enslavement in Encounter at Farpoint). It's presumed that anyone joining the Federation will abide by the Federation Constitution and/or the Federation Charter. From DS9:Accession
SISKO: You realise that caste-based discrimination goes against the Federation charter. If Bajor returns to the D'jarra system, I have no doubt that its petition to join the Federation will be rejected.
Valorum has a list of denied Federation applications