Tanks rely on positioning, and Mutalisks are traditionally used to pick off stray tanks with AA support. With that being said...
Auto turrets: 10 dps. That's similar to a stimmed marine's dps. Of course, they can't be spawned en masse nor do they really have that fear factor which 30 stimmed marines deliver when they're dashing straight for Mutalisks.
PDD: Blocks up to 20 attacks. One by itself won't do too much, maybe save a few tanks. Now combine that with maybe another PDD, one Auto turret, and marines marching in, we can get spared tanks while Mutalisks do virtually little to no damage.
This is a highly unrefined idea. I thought of this after raging at Ravens in PvT games.
This will most likely eat up gas that could be used for tanks
This could probably drive TvZ into another tank chess war (huzzah!).