Skills

Hold ALT in-game and mouse to the top of the screen. You'll get a drop down of 3 skills to pick from your 'draft pool'. Can also change teams / duos partner in the top right.
The draft tries to give you at least one passive per level, and avoids any skills that were in the last 3 options.
Each skill can have 4 points into it, increasing it's effectiveness.
Press Shift to Shift.
You want to unlock skills. You might be smart and never unlock a skill even if you can afford it, but in that case, the game just pretends you unlocked a random skill until you pick one.
No amount of health or defense bonus will save you from a charged knife.
You can throw grenades and use Cluster Bomb with Force Field active. It works really well.