The biggest problem that I'm familiar with in relation to a long back is just that it can be much harder for the horse to really engage his hind end, reach under himself and round out his back to achieve true collection. It's something you'll likely be working against for most of your riding career together (I know that has been the case with our long backed mare).
I found a bit of info on Wikipedia to give more detail:
"Long backs are more often seen in "gaited" horses, such as Saddlebreds or Tennessee Walkers. They are sometimes, but not always, associated with long, weak loins. The advantage to a long back is that it is flexible, making the movement of the back flatter, quieter, and makes a smoother ride. Even horses that are not gaited often have a smoother trot and long strides, making them comfortable to ride. On the other hand, it makes it harder for the horse to lift or "round" the back to develop speed or engage the hindquarters for high levels of collection. It takes longer to develop the muscles in a long back, and they are more prone to muscular strain and swayback as they age." http://en.wikipedia.org/wiki/Back_(horse