The changes for Guardian Druids make me believe that Haste will be a more important Rage generator than Critical Strike. It now reduces the GCD as well as the cooldown of Mangle, which becomes more important now that the cooldown of Mangle can't be reset, and we now only get 5 extra Rage from each critical strike.
Meanwhile, the new Mastery is atrocious. It means that every other physical attack is going to be taken to the face with essentially no mitigation, which means unavoidable spike. Even the old Mastery, which was just an armor multiplier, was at least a reliable steady damage reduction, and it typically resulted in Druids taking less damage than any other tanks in heavy physical-damage fights. Besides that, we have a substantial hole to dig out of with the loss of our bonus to Haste and Critical Strike from gear, as well as Agility no longer granting increased critical strike chance. So I imagine that Guardians will continue to avoid Mastery on their gear like the plague.
That leaves the logical and likely stat priority as Haste > Crit > Mastery, and the attack priority will essentially be: maintain Faerie Fire (unless you have a plate DPS providing the Physical Vulnerability debuff) > Mangle > maintain Thrash bleed > spam Lacerate every GCD.