It's got me thinking: What if they were to split up? Sure each race will prolly still refer to themselves as being Horde/Alliance but not part of the collective Horde/Alliance. I got to thinking this after seeing the new intros.
Here's how I'd see 'em if they were to break off into separate factions:
Alliance
Stormwind/Gilneas: It's obvious Stormwind is playing host to the only remaining free Human (sort of) nation left on the continent after the Forsaken Invaded Gilneas. Varian's overall disliking of the Forsaken (and holding them personally respobsible for the death of Bolvar) made accepting the Gilneans back into the Alliance an easy choice.
Relating to the other Alliance Races: Weather they've still got the backing of Ironforge is a pretty good question given what's going on over there. Sure, Muradin's doing his best to hold things down but they are kinda on the brink of another clan war. The Gnomes are like the back sheep of the Alliance. Their assistance in both The Burning Crusade and the Fall of the Lich King seemed to have gone unrecognized by Stormwind. The Night Elves continue to be a valuable ally as the only race on Kalimdor that can challenge the Horde. The return of Malfurion Stormrage means Tyrande can focus efforts on dealing with the Orcs in Ashenvale and Stonetalon. With the Vanguard in Barrens the Night Elves could open a new front from the north and make things interesting. Varian will likely want to keep the Draenei close to him simply because they have experience fighting the Orcs in Outland.
Khaz Modan-Dwarves: The three Dwarven Clans are going through some issues with the new council and all. The Dark Irons are split between those loyal to Moira who join the Alliance and those loyal to the Twilight Hammer who seek only war. If the Dwarven Alliance doesn't tear itself apart, it'll be that much stronger in the long run.
Relating to the Gnomes: The Dwarves' inviting the Dark Irons into Ironforge left their Gnomish cousins as the odd men and women out. They were leery of the Gnome Warlocks (more so after one "accidently" summoned Lord Jaxxus at the Argent Tournament) anyway so kicking them out must have been seen as necessary. Weather or not their relation to each other is effected remains to be seen.
Khaz Modan-Gnomes: They were kicked out of Ironforge. Geblin says "Meh, we're going to retake Gnomeregan anyway." Obviously, the boot from IF sped up the need to retake their capital and so far they're about 25% there (if I had to guess). There's no deny the Gnomes' technological ingenuity so they know they'll always be needed in the grand scheme of things.
Relating to the Dwarves: Can't help but wonder if there's a little bit of animosity between the two races. In part because they were kicked out of IF but also because of the unrest the Dark Irons brought to Dun Morogh. I get the feeling the Gnomes might have worn out their welcome at some point but the loss of Magni and Moira's return were too much to handle along with the Gnomes' situation.
Night Elves-Teldrassil: The return of Malfurion as a leader in Darnassus allows Tyrande to focus resources on dealing with the maurading Orcs in Ashenvale and Stonetalon as well as the Goblins who've terraformed Azshara. The fact that she accepted the Highborne back into Darnassus--powerful Mages who were responsible for introducing the Burning Legion to Azaroth--shows the Night Elves are willing to be more open and have come a long way from just after the Third War.
Relating to the Highborne: Although the Highborne were the ones who lured The Burning Legion to Azaroth and led to the destruction of The Well of Eternity (as well as the creation of The Maelstrom between the continents), the Cataclysm and several other factors have made inviting them back into the Teldrassil vital to the survival of the Kaldorei. I doubt Tyrande and Malfurion will ever fully forgive them (more so Malfurion after what happened to Illidan) but there's no denying the need for Elves proficient in the Arcane on Kalimdor.
Night Elves-Highborne: Ten Thousand years ago, they were the elite magic users of Azaroth. That all changed when they made contact with The Burning Legion. Their lure of power and their aggrogance nearly destroyed the world. The Night Elves' victory over the Burning Legion was not without cost. The Well of Eternity, Kalodorei's fount of power was lost to an world-shaking Cataclysm. Ten Millenia Later, the return of an ancient enemy to Azaroth has brought the exiled Highborne out of hiding. After meeting with Tyrande Whisperwind, the Kaldorei were allowed entry to Teldrassil and gave the Highborne permission to share their knowledge of the Arcane to the rest of their people.
Relating to Teldrassil: Azshara was the Queen of the Night Elves (then also known as the Highborne) prior to the First War. She and a number of her vassals disappeared when the Well of Eternity was lost but it's since been revealed she and her followers were transformed into Naga. The current leader of the Highborne has not yet been revealed (as far as I know) but whoever convinced Tyrande in the need for Darnassus to have them back.
Draenei of The Exodar: After the Burning Legion was defeated, many Draenei returned to Outland but the majority chose to stay on Azaroth and support the Alliance. Not much more than that is known but the Draenei have had a close relationship with the Night Elves ever since their arrival on Azaroth.
Overall: If the Alliance were to split, it would be to deal with internal politics. The Dwarves are obvious in this regard and possibly the Night Elves in dealing with the reintroduction of The Highborne. The Night Elves (with help from the Draenei) and Humans (with help from the Gnomes and Worgen) would still be able to handle things with the Horde on Kalimdor and Former Lordaeron, though.
Horde
Orgrimmar: With the new Warchief, it's obvious the direction of the Horde is uncertain. Unlike his predeccessor (Thrall), Garrosh doesn't seem intent maintaining a positive relationship with Sylvanas, Vol'Jin and the new Tauren Chief, Baine. He's not entirely opposed to diplomacy--he did welcome the Goblins of Kezan into the Horde, after all!
Relating to the other Horde Races: Garrosh dueled Cairne and is indirectly responsible for his death so he and Baine have a somewhat strained relationship. Sylvanas and Garrosh obviously hate each other and seems to have left the Trolls and Blood Elves to their own devices. The Goblins rescuing former Warchief Thrall led to their entry into the Horde and a piece of Orgrimmar to call their own.
Echo Isles: The Darkspear Trolls recently retook the land given to them by the previous Warchief, Thrall. They consider themselves loyal members of the Horde but it looks like Vol'Jin is playing a "sit an wait" game with the strained relationship between Orgrimmar and Thunder Bluff and The Undercity.
Thunder Bluff: The death of Cairne Bloodhoof was the perfect excuse for Magatha Grimtotem to try to seize power. Not only were her plans twarted but her faction was kicked out of Mulgore. The mantle of Tauren leadership is now on Cairne's son Baine, who must find a way to restablish a friendly relationship with the new Warchief and keep his people together.
The Undercity/Forsaken: With the defeat of the Lich King, Sylvanas Windrunner has been forced to play lapdog to the new Warchief. At the Battle of Anguarthar, HighApothecary Putress betrayed the Horde and unleashed the New Plague on the Alliance, Horde and Scourge forces gathered. The Undercity was taken over her Varamathras, who together with Putress drove her from power. Backed by Warchief Thrall, she retook The Undercity and slew the traitorous dreadlord...only to find King Varian Wrynn had come looking to retake The Undercity after defeating Putress. Although Jaina intervened by teleporting the Alliance Forces back to Stormwind, the Forsaken were blamed as a whole for the betrayal and war the King of Stormwind declared on the Horde. Sylvanas now finds herself doing whatever she must to show the distrust her people gained in Northrend from the rest of the Horde was falsely placed.
Silvermoon City: After Kael'Thas Sunstrider and The Burning Legion were defeated, Regent Lord Lorthe'mar Theron more or less is now the permanent leader of the Sindorei of Quel'Thalas. With the Sunwell purified, the plan is to cure the Felbloods, Wretched and other Sindorei who partook of the demonic energies offered to them by the Traitor Prince and The Burning Legion.
Overall: I could see the Horde break into subfactions--The Orcs, Trolls and Goblins as one faction and the Forsaken, Tauren and Blood Elves as the other. Unlike the Draenei, the Blood Elves were introduced to the Horde through Sylvanas. Based on that, it's likely Lorthe'mar would side with her both because of that and because they're close to each other. I could also see TB wanting to join with the Undercity-Silvermoon faction simply because of their distrust of the new Warchief. Knowing Sylvanas perfected the New Plague, which can kill both the living and undead he's not dumb enough to try to attack The Undercity. The Trolls would side with Orgrimmar simply because not doing so to them would mean to dishonor Thrall, who gave them the Echo Isles. The Goblins helped the Horde "upgrade" their naval power so...yeah.
Mind you, much of this is just what I've been mulling over. If the Horde were to split, the Forsaken would suddenly become the most dangerous faction on Azaroth simply because they're undead. At least with Thrall they kept themselves restrained to an extent but on their own, only the Argent Crusade/Dawn and The Ebon Blade would be a match for them.