Mist is the best skill in the game because it can mitigate more damage then anything (other then Gaycclimation).
It counters millionz of skills.
Fireball/RoI/Any stun/Pent Debuff/Bite Debuff/Deep Freeze/Ground Spikes/Slow Mines/Mark of the Betrayer/Boulder Roll/Hammer Slam/Tracking Device/Snipe/Mines/Spike Wave/etc
It lets you enter and exit battle once you get decent mana.
It can buy you time to get help
You can use it between cooldowns to mitigate more dmg.
The only real counter to it is Silence.