In addition to the Grand Canyon State, the beta includes some enthusiast-only new features. Physically simulated trailer coupling will put your reversing to the test—there'll be no more cheating by approaching the trailer at an angle, although SCS has promised it'll be optional. You'll also be able to take newly compatible World of Trucks contracts direct from ATS.
To opt into the beta, head to ATS in Steam > Properties > Betas > public_beta - 1.3 public beta. For the smaller additions (like XInput controller support), bugfixes and accessories, SCS has compiled beta patch notes, with the caveat that they may well change before Arizona goes live.