From what I know of Horde history the Orcs, Trolls and Tauren have gotten along quite well through pacts of honor and even friendship. The Forsaken withstanding, even the Goblins and Blood Elves have found a bond among the others within the Horde. It seems quite obvious how they've gotten along so far, despite the Forsaken and Furor Garrosh.
When it comes to the Alliance however, there's a lot that doesn't really make sense to me. Night Elves primarily were never really fond of anyone, believing themselves to be superior to all other races, and even after a huge explosion of humility, I find it hard to believe that such powerful, traditional, and reclusive beings could ever truly get along with "inferior" races, only survive among them. Politically, I can imagine the Night Elves feel more like prisoners than allies. I can see the Humans, Dwarves and Gnomes getting along, even before they realized they were all born of the Titanic constructs and cursed with flesh. They have similar goals, values and like it or not, they share physical characteristics. And the Gilneans, like the Night Elves who oddly cared enough to help them, have always been secluded/separated from the Alliance and are again only united by the need for survival, not a true bond. In a political landscape, Genn Greymane is no different than Vladimir Putin. And again, using the Night Elves as the entry point into the Alliance, the Draenei have only survival and common interests to become part of the "alliance". The only reason they didn't join the Horde was because Orcs.
So, the alliance stays together because of mutual interests but how come it appears that they get along so well together socially and politically?