It's funny because politically/socially, the Horde and Exiles are very similar. The Alliance and Dominion are far less similar to each other, but are more similar to each other than the Dominion would be to the Horde from a political lens.
Then you have the cultural/aesthetic lens which has them reversed. The Exiles' races remind me a lot more of the Alliance's races. Humans don't really have a sister race in the Wowverse socially but aesthetically, they're human. Granok are a blend of tauren size and toughness with a dwarven outlook on life. The aurin match up very well with night elves, who are nature lovers but also savage fighters. Aesthetically they're not far off from each other either. All in all, the Exiles are a bit more mixed but feel more Alliance to me in racial makeup than they feel Horde.
The Dominion reflect the Horde's racial composition really well. Aesthetically and culturally, cassians scream blood elf to me. Mechari are cold and calculating like the undead, except that their focus is on mechanical tech as opposed to biological or chemical tech. Then you have the draken which are a near perfect reflection of WoW's orcs and trolls.
An odd composition of elements in both factions for sure. When it comes down to it, people will go for the faction that has the feel they want the most. In most cases they'll refer to this as being the "Horde" faction or the "Alliance" faction, but when people say this, they're talking about the vibe they get from the faction being similar to one they're familiar with before. It goes a long way to explain why people are so divided on which faction in Wildstar is the "Horde".