I played a fair bit of my early Rugby at 12, before moving into the 10 position when I was about 15. As a 12, your all round game needs to be strong, and your game understanding superb. Playing 12 gives you that little bit more time to look around for options, and it's important that you give your 9 and 10 another option in midfield. Your ability to run decoy lines is absolutely crucial, and in order for your outside backs to function, you need to make sure your doing your job properly in that area. However, as pointed out, your main role is to offer a second phase - again, to give your outside backs decent ball to work with. You need to be comfortable in contact, but as I said earlier, you need to have the footballing ability to offer an alternative option in attack.
Defensively, you have a huge role, and it's true when people say that your 12 needs to be a 'defensive captain'. You need to be organising things, and along with the 15, making sure that players are in the correct position. As I said, you need to be an all round Rugby player, and you are a vital part of your teams attacking potential.