Nice feature. VVith the proper avertisement, this will surely attract new players. I've read some people's posts in the past, who found the 7x XP rates to be too fast, so now they can customise the game to their liking.
I'll still keep my 7x, but it's nice to have this feature implemented. So, good job!
Experience rates, while set to x7 on Frostmourne appear to only be base xp gains. In Borean Tundra or Grizzly Hills (wherever I quest) I only receive ~20k experience per quest. When doing the quest line for the Murlocs in the Tundy only one of the quests in the questline gave 140k xp. I don't know if this is because of an issue caused by the new feature or if it's just being weird. This issue is consistent with any of the characters I am able to level on Frostmourne all while xp rate is set to x7.
Made a new alt on Lordaeron just to play at x0.5! I hated how I got overlevelled fast, even if I didn't use heirlooms. With x0.5 it feels a lot more blizzlike when for example doing Outland (since Blizzard lowered exp required by 50% each expansion for older content). No more being level 64~ when finishing Hellfire.