As the most well known race in the Azeroth world, the human beings in WoW are a species native to the Eastern Kingdoms and are a key member of the Alliance. Famous for being the most resilient breed, they bravely fight through horrible times in all the land. Even after being almost eradicated by the Horde in the first great Orcish invasion, the humans managed to escape the genocide by the savage Orcs.
During the Second war, legions of armies of the allied Alliance rallied together to finally overcome the burning legion. This great success was not possible without the aid from all of their strong allies. The major ones comprise the Night Elves, Dwarves, and Gnomes who all are regarded as their fully dedicated allies in the Alliance. The fact that the humans do not make any remarkable progress in any one specific class is true but they are well-adept in every class.
And this is really the main reason to hugely help make them one of the strongest races in the homeland of Azeroth.
Even though they are not the best warrior comparatively to some other classes, for instance, they own the superior ability to train mages which can easily turn the tide in a battle against only warriors. Starting off in Elwynn Forest of Stormwind, the humans possess a multitude of unmatched capacities deriving from their racial traits including Perception, the human Spirit, Sword Specialization and Mace Specialization.