It depends, too, on one's definition of 'awful' and how high one sets the bar.
Some might think a DM is awful just because she neutrally and fairly runs a lethal-to-the-characters game and they don't like losing their characters.
Some might think a DM is awful just because he won't allow their favourite character species-classes-concepts into the game for whatever reason.
Some might think a DM is awful just because some unpleasant things (slavery, colonialism, racism, etc.) are portrayed as being accepted norms in the setting.
Thing is, even though they might not be to a specific person's taste, none of these make a DM awful. And yet even in this thread I've seen pretty much just this: examples of awful DMs where further inquiry shows their only "fault" was one of the reasons above.
There's a huge difference between an awful DM and a DM that for whatever reason you'd simply choose to not play with.