After patch 5.4.2 was applied, you may have noticed something like this:
Notice anything missing?
No matter who it was, the nameplates no longer showed the names, making them just ‘plates’. If you used something like TidyPlates you might not have this problem, but there were a lot of other people that did. What was up?
The first clue emerged on the forums – if you had Tekkub’s Tekticles installed, which modified the fonts used in game, you might have this problem.
But if you weren’t using Tekticles, and still had that problem? Look for an app called BetterFont – "!BetterFont" on your addon panel, and thus near the top. If you have this installed, disable it. You’ll probably no longer have the problem.
If you have neither of these, search out other font-altering addons. If unsure, disable them all, and then enable each one, one at a time, until the problem occurs again. That’ll be the addon causing the issue.