So, a couple of my 'Murican chums have already DM'd me to say Tanaan is garbage.
The thing is, I don't think such a short period of time is enough to make a reasonable judgement of anything. I suppose you can spot bad sex after about five minutes, but even that's assuming the problem isn't you.
I think the problem is that people don't really care about Tanaan. It doesn't feel threatening, the Iron Horde were impotent throughout this expansion, and players are just asking themselves what the point in all this was. In game terms, I've literally got no connection to the world my character is in; I can't relate to the warlords the way I did before, because they act in ways they hadn't previously (with the exception of Gul'dan), and the story just didn't develop any character to the point where you cared about them.
Even when Velen died, it was like:
"Oh, that's AU Velen - not OUR Velen".
I think this general malaise with the story in Draenor, including it's extremely anemic character list with truncated development arcs, has made players wonder why they're bothering. The game doesn't feel like an adventure, which means when you get to Tanaan you're already looking past it.
Maybe it's time Blizzard started putting the story and adventure back into their game about the stories of adventurers.