A few joker tickets, a title or at least a hat or a shirt. Right now there is no real reason to try out the beta given that the performance is mostly the same as the last one and downloading the beta client is difficult for many of us because of the slow download speed of the launcher. 2 or 3 players online is not enough to properly test the game (assuming we eventually reach a point where it is playable without droping to 15 fps every minute)
I really hope they don't remove this with the engine upgrade. My gf has ectrodactyly, and this is the only thing that makes the game playable for her, given that she can press only 2 keys in the keyboard at a time.