You may know this already but I just found it out via testing with FTC damage logging.
If you cast a damage shield and then activate Mist Form. The damage your shield takes is reduced by 75%.
I have a high magicka regen sorc about 2500. I've been trying out different play styles and found this out. I was damn near invulnerable in pve. I have yet to test this against big bosses so I'm not sure how viable it is for tanking or attrition fighting. But I think it's worth further experimentation.
I had my pets up for damage, cast lightening form, velocius curse, hardened ward, healing ward, then cast poison mist. just as poison mist expires the healing ward pops and velocius curse explodes. Then you can curse, ward, and mist again. If you need magicka again and the boss/mob uses magic you just throw in the light armor shield for magicka absorption.
You can't cast or heal while you are misted so pbaoe pulse and DOTs are your friend.
I also found that swapping bars breaks mist form so you can mist, swap cast heal/damage/buff then swap back and mist again. You can do this really quickly