To be fair the small-scale dialogue tends to be utterly fantastic. Like the stuff between your followers or townspeople, etc. Its only stuff related to the main plot itself that seems to be ass.
My theory is that there is a group of relatively competent writers at Blizzard. But Metzen is the creative director. He is blizzard's very own George Lucas. He was good when he had other people around him to bounce ideas off of. But now hes so high up that no one is willing to say no to his bad ideas. Thus we get the plot of SC2, D3, MoP, etc. which still has good writing, just not from the main plot itself.
At least thats how I try and rationalize the quality disparity from their late 90s games compared to now.