Agree with all the people saying Dagg, Ioane and Jane. Dagg, like O'Connor is just starting his career as well. He is going to be an absolute star.
My first pick though would be Nonu. With all the controversy about him being off contract earlier this year I was really hoping the Warriors would make a genuine attempt for him. It made a lot of sense because the Warriors would probably have enough cash to do it, they really need a quality centre, and by 2012 it was probably at least even money odds that Nonu would have a World Cup win under his belt so he wouldn't have that reason to stay in union. He really has all that is needed from a league centre: pace, power, an amazing ability to beat a man one-on-one, and a distribution game that has improved out of sight over the years.
The other person I would nominate that hasn't been mentioned yet is Dan Carter. Pinpoint kicking and passing game, and perhaps most importantly for a union half converting to league, a real talent at taking the line on and choosing the right times to do so. This one would never happen though unfortunately, he's one of those guys that you perceive as 100% union.