One common design in Warcraft is that the Horde and Alliance have always, always been mirrors of each other. From gameplay to thematic. Similar, but with differences.
This is after all a game where you might end up playing a faction you're not a huge fan of for the sake of playing with friends.
Just as a quick example in thematics, both Draenei and Tauren are very close to each other thematically. They're both good-aligned, hooved races that tend to be larger than their peers.
But what about themes of darkness, and their interest? For all intents and purposes, I'm not talking about "evil". Evil would be which faction kidnaps babies and eats souls. That's totally Horde. I'm talking about darkness. Darkness is not necessarily evil.
For the Alliance. Here you have Night Elves, who, you know...are nocturnal. They tend to be a bit extreme in how they operate, and can disappear into the shadows. They're strong, but tend to prefer intelligent tactics over brute force. Like all Elven cultures, they have fanciful buildings and scenery. My only complaint about them, in regards to this topic at least, is that the "night" motif of the race all but vanishes after Ashenvale. Hyjal does not have the same dark sky look of Ashenvale or Darkshore and as far as I know the zone only has, like, one barrow den.
Then you have the Worgen, which is essentially the Alliance's equivalent to the Forsaken. These guys are ferocious, ravaging beasts who look like they can tear limbs off. But they're also just regular people that happen to have a violent side that is more violent than most. They have dark buildings and Victorian themes, and their theme song is great and moody.
For the Horde. Here the obvious "darkness" race is the Forsaken. They care very little about honour, and do not shy away from the darkest of magics. They will melt your skin off with poison before they engage you in combat, and they will trick you with lies and deceit before they speak to you plainly. Forsaken architecture is a mix between gothic styles and Frankenstein/Tesla references. Though Forsaken areas tend to be very moody(and gooey, for some reason), they don't seem to have much in the way of a compelling musical theme.
I would also argue that the second "darkness" race of the Horde is the Trolls, even though they have next to nothing to do with the Forsaken. The Darkspear only gave up cannibalism and other voodoo practices when they joined the Horde. And though their visual styles tend to be very bright and colourful, as a rule they tend to often use their own style of primitive magic. And that's where the "darkness" lies--they are from a culture that for generations has practiced blood magic, sacrifice and the worship of spirits that are not necessarily benevolent. Even going by the tribe's name alone, they were never meant to be a necessarily cheery group of Trolls. Or to paraphrase Blizzard, "Most Troll tribes are evil, but the Darkspear are the ones who are trying to steer straight".
So which of the factions(or races, if you prefer) represents the "Darkness" theme the best? Since this thread is obviously all about opinion, I'm just going to put this out there:
Sylvanas could kick Tyrande's !@# any day, no matter who is writing Tyrande. Elune's hand maiden lost my vote when she got the Peggy Bundy haircut.