I think this is the closest real-world analogy we have to them
So, we have two unions of vastly different nations and even races. Which have rather different interests, but are united for the sake of the benefits they get from the union (stronger economy, military, etc).
Not surprisingly, they face the same problems as the EU:
1) The decision making is hard, slow and very inefficient because every race has different interests and objectives
2) Races sometimes want out since their interest are vasty different from those the whole union (UK/Tauren)
3) But they don't really want out since that will bring even more problems
4) The Warchief (and whatever the alliance leader is called) is not all-powerful - similar to the EU commission. While the warchief defines the main war direction and objectives, every race is more or less pursuing thei own interest in the undertaking, which often contradict the main effort and creates a lot of conflicts along the way.
For example, in Northrend you could clearly see the Orcs moving in from the Tundra for a direct assault, the Undead starting at the Fjord, and the Goblins testing out their shredders and whatever else they have as well as moving into the Storm Peak.
5) Finally, there are no universal laws across the Horde or Alliance, so something that one race might consider good or appropriate would be completely unappealing to another one.