So, if you're one of those obsessive types who, the second an expansion drops, is off to the races trying to nab the exclusive realm first achievements... you can relax. For Owen Landgren, Items and Rewards Designer for World of Warcraft, has tweeted that such are a thing of the past.
@Seerax Realm Firsts for leveling are removed in Warlords.- Owen Landgren (@olandgren) February 27, 2014
Yes, this means that you can't get the bragging rights for being your server's first... whatever you are... to 100. Don't even try, because it can't be done. Well, you could theoretically still be your server's first Monk to 100, but those sweet, sweet achievement points will no longer be awarded for it. So you might as well do what I'm going to do and wait a day.