Changes over time will probably get tweaked or adjusted, some of the special achievements/reputations have been mentioned as not being warband wide.
Yet things like making a Nightborne character you can't use the portal to the Surmar Nightborne neutral faction base until level 45 yet getting to there even a lower toon Nightborne gets auto ported into the unlocked version and being exalted that character can get the Shoulder enchant as one key example that other characters have to grind reputation for.
I believe that this faction will not get auto-Exalted via warband. Yet again this could change, as it is "new" Blizzard.