And for a fun fact, if you play horde, go to the throne room above the UC bank, and turn down music while upping ambient noises... I guarantee you will be disturbed, if not intrigued. It takes a while so be patient.
Just a quick note:
Night Elves main enemy were always the Burning Legion, the Orcs only became their enemy when they started chopping down their trees, oh and when they murdered their friggin god.
Dwarves and Gnomes kinda disliked Trolls, simply because their tribes pushed into their kingdoms, and also they made babies with something or other and spawned TROGGS!!!
Humans hate orcs and trolls for halting their advancement as a race and nation.
Dranei hate the Belves (well more like distrust) because of the whole Outland incident, which coincidentally applies to Orcs butchering the space-goats mercifully. And yet now they can slay gods, aspects, and other monsters stronger than your average orc scout...
thought i could try and make sense of what few things i got from lore discussions. oh, the blood elves get the sunwell back = not good for ANYONE. Just saying, the belf ties to the horde were for protection and vengeance at least. with the power of the well on their side... who knows.
Kingdom of Lordareon wasn't attacked by Archerus and Naxxaramus before the Scourge. The plague was spreading extremely fast killing and turning the citizens into Scourge as well as Arthas tearing through the kingdom to get the jar his fathers ashes were in to put kel'thuzad in while the scourge was becoming an actual Army.
Night Elves brought the Burning Legion to them using magic and then blew the world up and yea Grom did kill Cenarius.
Humans helped the Quel'dorei(High Elves) fight the trolls before they become the Sin'dorei(Blood elves) and then the humans were taught magic by them. Also I don't think the trolls made life for the Kingdom or Arathor easier.
Also "Mercifully" isn't really the word to use for the Old Horde killing the Draenei since...the path of glory in Hellfire peninsula is paved with Draenei bones