I am writing this because I have played in the last couple of days with several new players and I see always the same mistake. I hope this helps.
What is a quick deck?
a deck which plans to kill the opponent before he gets control of the board
Golden rule for quick decks:
Opponent needs time to place his strongest creatures on board. If your creatures are not that big and quick you can kick his ass before he plays important cards.
what is a control deck?
a deck which plans to get control of the board before opponent kills you
Golden rule for control decks:
You have 20 life as buffer. It makes no difference if you lose 1 or 19. It makes difference only if your life is set to zero. You can take damage and get an advantage for it!
Subvariants of the game:
Quick vs Quick: wins the quicker
Control vs Control: game usually goes on stall
I assume now you have very clear in mind the concepts above.
I am playing a quick deck and opponent, with 10 life, places a lavaworm ravager (2/5) on board. I have seen many players that would place a giant owl (3/1 quick) on the same path of the lavaworm. THIS IS WRONG IF YOU WANT TO PLAY QUICK!
Would you do the same if the opponent had only 3 life?
Remember: the goal of this game is not destroying opponent minions. It is damaging the opponent to zero life which is a consequence of it, but you do not need to destroy all of them!
There is something you have to sacrifice in this game to win. It is not always your minion. It can also be your life points. You must know what is relevant to win and what can be sacrificed to win the game.
If your deck is a quick one it is wise to accept the damage if you can damage as well when:
a) opponent has less life than you
b) you can cause more damage to opponent than the damage opponent can cause to you
If your deck is a control one it is wise to accept the damage when:
a) your life points are still high
b) it allows you to get a better control position on the board
Would you sacrifice an aquamancer to protect you from the aeromancer attack at the beginning of the game when you still have 20 life?