If you finish the scenario Theramore’s Fall at exactly level 35 you get the original Feat of Strength from the MoP-pre-patch for doing the scenario. This qualifies all of your Alliance characters to either buy the tabard from Zirodormi at Theramore or get it from the tabard vendor in Dalaran. It doesn’t matter whether you do this on a Horde or Alliance character.
If you do the scenario with any other level than 35, you only get the regular and remix achievement for finishing the scenario but not the Feat of Strength!