I've finished all the quests for the guild. I now only have the jobs available (burglary etc) but I'm still not yet leader. I haven't had a ceremony and I haven't unlocked the achievement!

What do I do?

You have to complete the random jobs and gain influence in the holds. Once you do that, you get a special mission to gain an ally in the holds. Just finish all of those, and then you can become the leader of the Thieves Guild. This can take a while, though, since the location of the jobs are random.

Do 5 jobs in every city [from Delvin and Vex], which will unlock a "special quest". This will unlock it, I believe.

You have to do about 20 or so sidejobs and 4 hold specific quests, then you get to be the head honcho

5 random/radiant jobs from Vex or Delvin per major city (minus Riften, makes 20).
Then 1 scripted quest per city.
Then tada ! you are guild master and are called boss

EDIT : This also allow shops to open near the Ragged Flagon with of course up to 4 of them and also increase up to 4000 the available cash at Tonilia the fence (wich makes her the best shop in the game as far as i know since she buys anything stolen or not and in large quantities).

You guys are awesome! Thankyou