Probably not viable for all classes but it seem to me it might be very viable for some.
Some obvious choices here like:
DPS two-piece Your melee auto attacks have a chance to trigger Enrage.
DPS four-piece Activating Skull Banner now also increases your critical strike chance by 35% for 10 sec.
Two-piece Increases the duration of your finishing moves as if you had used an additional combo point, up to a maximum of 6 combo points.
Four-piece Shadow Blades also reduces the cost of all your abilities by 40%.
Two-piece Predatory Swiftness grants you an additional combo point on your current combo target when consumed.
Four-piece After using Tiger's Fury, you gain 40% increased critical strike chance on the next 3 uses of Mangle, Shred, or Ferocious Bite.
Don't know too much about the other classes in MOP so don't know how good all set bonuses are.
The warrior one is another recklessness burst cool down and the Rogue 4 piece is also a very nice burst CD during shadow dance and the two set bonus will mean longer kidney and cheap shots. Seems really broken to me.