Fifth: Well the DM is the final say so as to what goes on in his/her game.
That's one model. But it's really an illusion.
RPGs are social activity, and absent force, all social activity is a result of voluntary collective action.
A bunch of players get together to game. The result -- and the protocols involved -- are a result of the agreements, implicti and explicit, that they make for structuring their activity.
You don't need a GM (and there are plenty of GMless roleplaying games; they work fine).
If you do have a GM, they need not have rules authority.
If you have a GM and they have rules authority, that authority is -still- only as binding as the other players will let it.
The GM, ultimately, has the power to refuse to run (eg, leave/stop playing) unless a player accepts their authority or leaves. A player has the power to leave or argue in order to persuade others -- the GM or other players on their side.
Ultimately, a GM is no more irreplacable in a game than any other player is. Each player added or gone significantly changes the resulting game -- but play can continue, with the same apparent world, the same characters, and same overall situation with any player replaced -- even the GM.
Maturity, for gamers, means playing the things you want to play and not the things you don't. Ultimately, that means spending enough time working out the ground rules for a game that it's something you'll find fun, and working out things rather than rolling over and taking it when things aren't working out.
Sometimes, that means the entire group hitting the GM with a cluestick when the GM is being dumb. More often, it involves more subtle negotiation. But gaming has progressed a lot from "the GM is in charge of everything except the characters; the players control their characters."