If your benchmark for taking a villain seriously is Deathwing, then yes, I can. Given Deathwing murdered a ton of people in the pre-patch event and then just sort of plateaued from there, not really showing up again until the last raid, where his big ultimate plan was to fly to the center of the world where he had started, I can take pretty much any villain that seriously unless they opt to make Budd or Silvermoon Harry the next big bad. While the Lich King was a lot better as an expansion villain, we cannot forget the true final boss of that expansion: Halion. That makes Deathwing the only time the final boss has been the primary enemy from pre-patch to conclusion, and as I mentioned above, the story had some flaws.