Oh, in that case: I think what fucks hella amounts of people up is the adamant refusal to pick up cards that don't really relate to a "build." Like not picking up Sly or Poison cards on a Shiv build, or only picking up Forge cards on the Regent.
It's fine to just grab good cards that are good. Like Preparation (Silent), Stardust (Regent), Defy (Necro), otherwise you're just going to create an inflexible deck that can't really adapt to difficult circumstances. Also, you want to grab 1 or 2 AoE cards early. That's big.
So if you're going an Ethereal deck on Necro, Queen and Doormaker are gonna fuck you up big time. You want to splash Soul or Osty cards as an alternative plan if Ethereal ain't gonna work.
Outside of that...I guess just don't be afraid to take damage. The big fucking Byrd elite in Act 1's a good example of this. If you keep blocking, it's gonna scale, and you won't be able to kill it before it kills you. If you need to take damage to attack, sometimes it's better to do just that. It really depends.