That kind of error is exactly why the beta process is necessary and important, but moreover it's why it's kind of awesome - you see, stuff like this is terrible for game balance, but it's also astonishingly fun to discover and experience a few times. Yes, as a tester you should absolutely report it as soon as possible. But in order to report it reliably (it's hard to know for sure what's actually happening, after all, your combat log doesn't report that you're taking less damage) you need to repeat it. And that's really pretty awesome.
We discovered a teeny tiny little issue with the Battle Fatigue debuff. Turns out, it was set to increase tank damage taken by -25%. Yes, that's an increase of negative 25%... which means that Battle Fatigue was actually reducing the damage tank-specced characters were taking.
This should now be fixed on Beta servers, which should make quite a difference. Let us know how things feel now.
It's better -- far better, in fact -- that we get this stuff found out before it goes live. Even though I'm sure all of us with a tank spec kind of secretly wish it didn't. The feeling of being overpowered wouldn't keep us from the crushing disappointment when they found and fixed it.