Nothing. There's very little incentive to play.
You can get all the codexs but what do you get from that? Bragging rights that nobody cares about. You can finish all the challenges but what do you get from that? Bragging rights that nobody cares about. BW should give an incentive to finish codexs and challenges...maybe a power special ability or power special weapon.
The game goes like this: level toon to 55, gear up...than play for hours where your character doesn't get any better. And it's so easy to max your toons ability out. There's no character progression after hitting max level all there is to do is go after bragging rights: codex, challenges, cool looking gear that does the same thing as what u were wearing before.