Which races in the Alliance and the Horde, and other organisations, do you think were let in or let back too easily despite a bad past between these races and their new allies and/or a more or less well-deserved reputation and other factors indicating untrustworthiness that should have made it far more difficult for them to be accepted or accepted back ? What should these races or groups have done to prove at least that they could be trusted to an extent or that their usefulness and power were worth the risk of them joining or rejoining a faction ?
No doubt that, even with it being said that Thrall and many orcs were relucdant at accepting the Forsaken in the Horde and that they didn't really trust them, there should have been many more signs of distrust and wariness from the Horde toward Sylvanas Windrunner and the Forsaken in general, especially after the Wrathgate and the discoveries of the Blight and of the abominable things done and created by the Apothecaries in the Undercity, and that more measures should have been taken to make sure of the Forsaken not betraying the Horde or getting away with their actions in the story than just posting Kor'krons in the Undercity. No doubt that it should have taken more time for the Horde to actually accept them as official members of the Horde, with the Forsaken doing things to at least present themselves as valuable allies before.
Similarly for the Alliance, the Dark Iron Dwarves should have been accepted with far more difficulty and wariness by their new allies, given the Dark Irons' reputation for cruelty, treachery and wrathfulness, as well as of their deep ties with Ragnaros, their uses of dark magics and their hostile history not only with their fellow dwarves but also with the humans of Stormwind and the gnomes of Gnomeregan (with Dark Irons having ties with Sicco Thermaplugg). It should have taken far more time for them to be fully trusted, and them doing more to justify their return to Ironforge and entry in the Alliance such as them proving their worth in several battles (for example in my battle of Stromgarde scenario, I see this battle scenario as it being a test to see if the Dark Irons could could be useful and trusted against the Forsaken and the Horde, with the Dark Irons proving their worth with their golems, fire and shadow magics and own dirty tactics being very useful against the Forsaken).