There's lots of different forms of immaturity though. The one that irritates me most are those who take games far too seriously - which your example certainly does, but there are other strains. Frankly, there's no escaping people who irritate you; the only way to deal with them in an effective manner is to grow a thicker skin. If they are so toxic that you simply can't be around them, then ignore them. Trying to push the players into behaving to fit your particular comfort zone through game mechanics, however, is a recipe for disaster, both in game numbers and divergent play.
Of course nobody is forcing their point of view upon them. But there's a difference between lacking proper education/being a scumbag and actually being funny. I don't think anyone here would stick to such things as "don't tell any jokes, don't break RP, I'm grimdark and cold". Then again, I prefer RP to mentally impaired 14-year old CoD players.
Had that issue before BUT it's probably reduced in this as to get high ranks like Autarchs you need votes to get them. Plus seeing as we have friendly fire there's going to be a LOT of accidents around me.
He who must not be named!!!!!!!!!!!!!! As for the toxic elements and such, that just comes with any MMO, perhaps all that is needed is a decent report function or even better, an easy to use mute function.
The suggestion that the elected squad leader should become the squad Exarch has gained a little merit with OP's point.
The Eldar force is likely going to be a close section of the community due to our low numbers so if someone is enough of a jerk anyone who pays attention will end up knowing who they are and will not come to help them when Chaos come to show them where they will touch them on the doll, so they will likely just change to a either Orks that has the f2p player base that will allow such actions in its own team or CSM and SM who will have the numbers for it to still be common even if it would be looked down on so they will need this more then we will. Lastly when it comes to "Noobs" I personally don't care about an allies skill level as long as they are trying.