This so much.
They were interesting when we barely heard about them. They were mysterious and terrifying. And then we got more with the lead up to Yogg and the battle with Yogg. It created a mounting sense of wonder and dread. Great.

But since then, the Old Gods are just... there. They have no impact. Hell, the naga are fighting alongside Faceless Ones (which actually HAVE faces, what??) in Vashj'ir as if they're just normal soldiers. One does not simply put Old God forces in the world as just run-of-the-mill enemies like that. Old God presence should always have impact, from Faceless Ones all the way up to the Old Gods themselves.
Remember Herald Volazj? Creepy and unsettling. Then we get gigantic maws erupting from the earth in Cata and they're practically just dealt with like some normal threat. Yes, there's the whole nightmare of Iso'rath business, but let's rewind. I first saw the thing (Alliance-side) while questing for the dwarves, who seemed wholly unconcerned by the giant, spiked, flailing Cthulhuan tentacles in their backyard, which I found incredibly surprising. Eventually the player finally gets to deal with it, but we just... kill it. Then we leave. No concern as to how it got there or how to get rid of its corpse. No fear. No contemplation. Nothing. Thus, no sense of narrative/emotional impact. Bah.

Well this turned into a rambling rant. I'll stop now.