Mastery is my favorite stat in World of Warcraft because it's the only stat that actually changes function from class to class. Haste might speed up rune regeneration for death knights, or fire mages might view a huge critical strike chance more happily than an affliction warlock, but those stats do largely the same thing across classes. On the other hand, the mastery of an elemental shaman is incredibly different than a holy paladin, which is a big change from a protection warrior.
Lucky for me, this patch gave rise to mastery as elemental's key secondary stat after hitting the spell hit cap. It's been a good stat all expansion but sat solidly behind haste through all of Firelands and much of tier 11. With Dragon Soul out and most ele shaman who have been playing the past few months now wearing their tier 13 sets, it's mastery's turn to shine.
The reason for mastery's newfound glory is the Spiritwalker's Regalia four-piece bonus, which gives you 250 haste rating every time your Elemental Overload procs, stacking up to three times and lasting four seconds. At a full stack, this is effectively 750 free haste rating that you get just for gearing for mastery. However, this stack falls off quickly, and chances are you'll only get one or two spellcasts off before it drops. As a result, you want a lot of mastery, to give the two casts you do get a fair chance at keeping the buff up.