I would say you will need a certain skill to advance in rank, not to join. this to keep 30 intellegence orcs from becoming arch-mage.
I agree with this.
And not just a dumb Orc becoming Arch Mage, but in TES IV: Oblivion, that Orc can be come Arch Mage without using a single point of Magicka. There is only one quest in the quest line were you actually have to cast some spells, and they can be cast with scrolls that are provided.
Also agree with the above post that for some of the higher ranks it would be nice if you had to demonstrate your magic skills with some test or puzzle.
Similar things can be said of the Fighters Guild and other guilds. In Oblivion a wimpy mage that's never worn armor on punched someone can become the Master of the Guild, without ever using a weapon.
It would also be nice, in my opinion, if there were conflicting guilds - sort of like Morrowind - So that you couldn't become the leader of everything. That would result in meaningful choices with consequences, which I think are a key part of RPGs.