It's a good question who controls the bitcoin. I think a group of dev controls it. Because for bitcoin, there are some private keys (for checkpoint and alert) that if one has it, he will be able to propagate his own blocks into the network. I think a few people hold the private keys there.
There's actually a website that has the private keys of all the bitcoin addresses. Funny because even if you do have it all saved on a .txt file, if will take you a lot of power and time (even years) before you can find the private key of a specific wallet. There have been discussion about this in this forum. There are just too many bitcoin addresses available and I really mean way many for you to be able to control bitcoins or hack someone using the list of all private keys.
I don't think you get what marseille said. He meant that in bitcoin, there are two private keys, not for wallet, but bitcoin itself. One private key is for checkpoint. If you have it, you can define the mandatory checkpoint that every other client will accept. Thus you can propagate your own blocks. I know this is true for the old version of bitcoin, not sure if this is changed or not. These private keys are controlled by a few devs in the inner circle of the bitcoin.