If you have at any time set your WOW to work on either:

Window mode
Full screen (Window)

this is relevant to YOU.

If you play at 1080p the client glitches when u switch to either of the window modes.
Basically, even when you try to go back into full screen mode, your refresh rate will be lock at 30.

It will either be grayed out at 30, or it will let u choose something higher but when you click apply it will go down to 30.
A refresh of 30 is annoying, sure, not a deal breaker, but why put up with it?

QUICK and Easy Fix:

Go to your game folder. (Right click the shortcut and click on "Open File Location"
There you will see the WTF folder.
Inside that folder there should be two files: Config.wtf and Launcher.WTF

DELETE the Config.wtf file.

That's it!

Next time you run wow it will create a new un-glitched config file and your refresh rate will be unlocked again!