You get a second character.
To be honest, I just wanted to stop there – that's broken, right there. Stack Exchange, however, makes me write more.
Cohorts are almost as strong as PCs, and you get all this performance for the price of a lousy performance. Tricks are powerful and valuable, but if you have a different character, you get at least three, so you get +2 net for heroic deeds, and that literally ignores everything else about the character. Sure, the cohort is mushier and the division of performance between two characters limits the synergy literally the same as +2 tricks, but you get so much more. What are the most valuable things in the game? Feats? You get more of it. Spell known? You can get more of it. Magic slots? More of that. Actions in battle? You get more of it.
Leadership is so powerful that it is probably the strongest choice for 6th level tricks, even for druids who would otherwise consider Natural Spell a foregone conclusion at this level.
It is also a headache to decide. Technically, the feat requires a lot of DM input – which is a big question, since DMs are already pretty busy. And since the DM creates the cohort – the PC can only roughly describe the desired attributes such as race and class – there is one ton from the judgment on the line here. Exceptionally bad decisions on the part of the DM can massively reduce the performance of the feat, but that is also problematic. And if you're tough enough, you could turn the cohort into a liability that is just miserable.
After all, it's just … some kind of nonsense in the game. The cohort is not a servant like an animal companion, a familiar or special mount that has a magical connection to the PC and is expressly subject to the PC. The cohort is said to have a completely independent character. As such, you shouldn't be able to get one by spending just one achievement – you should have to play the role-playing game, and it should be handled accordingly, with the character making truly independent decisions and possibly the PC on some of them not accompanying activities. It may seem like an achievement, but it would make up for it, but it means that the PC "paid" for it and is therefore owed. It's a really problematic dynamic.