Stories like this are what I think of when people say that playing humans is too boring for a fantasy game. The vast majority of the other races are all pretty easily defined with broad generalizations, Trolls are bloodthirsty, Nelfs are isolationists, Tauren are peaceful, Dwarves are drunk, etc, etc. Humans however are shown as each of these traits and are at war with each other as much as the Horde. Everyone playing can understand living in uncertain times, seeing one war end only to have another one erupt while still reeling from innumerable other problems. Humans are complex creatures, unique even from town to town. Blizz makes it clear that the broad brush used to paint the other races just doesn't apply to humans.

As for strictly game-wise, my loyalty to Stormwind is because they stand united with ancestral kin against creatures out of nightmares. Orcs marauding from another world, trolls erupting from dark ruins in the corners of the jungle, the walking dead with their familiar faces and unrelenting hatred... It's not just familiar vs ugly, it's (from the human point of view) purely self defense against an enemy that would rather see you dead than have any sort of peace. Humans have been on the ropes since the first war, and have only lost ground since the orcs first arrived. So when I hit a BG and tear through my enemies, there's more behind it than just looking for some quick honor or proving that us old guys can still kick PvP ass.