It's not entirely clear at this point. We didn't have proof that all undead went there before and Thoras' words cause even more confusion as the living can end up in that dimension as well. Maybe being living or dead makes no difference whatsoever.
Warcraft is made of factions, it was from the very beginning. WC1, WC2 and BtDP were all about orcs and humans killing each other. WC3 expanded the concept to four factions by adding Night Elves and Scourge but for simplicity's sake they found a way in WoW to implent these factions respectively into the more iconic Alliance and Horde.IIrc wow was originally not supposed to have factions at all then they threw them thoeather because they wanted to drive pvp harder. The factions and need for balances severely impacts the story.
In the end Warcraft has always been and always will be a game about opposing factions, is what lies at its core. Sure, there's the whole PvP thing but this expansion is proof that regardless of PvP, Blizzard always tries to fuel faction conflict within the story itself. The only problem with that is how bad they grew in handling it, struggling to make said conflict credible and the motivations behind it grounded.