One: Coining Out Wrong
This is the biggest mistake that most new players make, even ones coming from other games. A lot of them think that the best way to play is to put down the biggest thing you can as fast as possible, but this is not the case, especially when you have to use the coin to do it. Coining out a two drop on turn one is almost always the wrong play if you don’t have something to follow it up with on turn two, worst case scenario is usually hero powering turn two. Instead, save the coin, play your two drop on turn two, and then have the coin available for a big four drop the next turn, or coining out your late game finishers early, something that will actually influence the game, not for a chance at an extra hero power.
Two: Overvaluing Taunt
This is something every new player does at some point or another. Life total as a resource is something that a lot of players find intimidating and seek to protect it at all times. However, taunt creatures are often understated for their cost, so by playing exclusively taunt minions you’ll find yourself simply being overwhelmed by your opponent’s stronger minions over time. Taunt can be very important, and if timed correctly, game winning. However, stopping your opponent from dealing two points of damage to your face is far less important than contesting their minion with a stronger one. Playing a Sen’jin Shieldmasta certainly isn’t wrong, but playing it over a Chillwind Yeti isn’t always right either. Use the taunt to protect your other minions and force your opponent to make bad trades, not because you are scared of three damage.
Three: Overvaluing Healing
This ties in directly with the second point, but is just as 
important. As stated above, most new players find the idea of a finite life total intimidating. They seek to remedy this fear by playing taunt minions as well as healing minions. Healing does not win games. It’s a difficult concept for some people to grasp, but at the end of the game, if your opponent’s life total wasn’t zero, it doesn’t really matter how much you healed. There’s no prize for winning the game with full health. Just yesterday I saw someone who thought that compromising an entire aggressive deck would be a good idea just to put in Reno Jackson. The idea is that aggressive decks kill their opponent quickly. When you’re playing an aggressive deck, if you get to the point where you need to heal or you’ll lose, you’ve already lost. I know it hurts, but take the Antique Healbot out of your Zoo Warlock deck!
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Four: Playing Everything
This is a difficult one to learn, and an incredibly difficult one to master, but you don’t need to play everything in your hand as fast as you can. Oftentimes, especially when you are playing slower decks, you’ll find it’s best to not play anything at all. For example, when I play Control Warrior, I often find myself using my hero power and passing the turn when I have eight cards in hand. Oftentimes it’s better to force your opponent to act than to act yourself. If you can see that you’re going to win based on the fact that you’re hero power is stronger than theirs, simply force them to play something so you can remove it. There’s no need to play into removal spells yourself, giving your opponent something to do. This is especially true when your opponent has eight or nine cards in hand and you have fewer, because then they are forced to play something sub-optimally to avoid discarding cards. Just slow down, holding your cards for better opportunities is often good.
Five: Playing to Win
This is easily the hardest point on the list to fully grasp. Even as a concept a lot of players have difficulty with this. Basically, if you play not to lose, you’re often not playing to win. Meaning that even though you may postpone the game several turns, you’ve put yourself into an un-winnable situation. I’ll give you an example. Your 
opponent has ten damage in play, and you have fifteen life. You are holding a Dr. Boom and an Antique Healbot. Now, there’s a good chance that your opponent has five damage in hand, so if you play the Dr. Boom, there’s a pretty high chance you’re dead. A lot of players will see this and think that the Healbot is the correct play because it keeps them alive for another turn. However, unless you’re postponing for something specific, like a board clear spell, you lasting one more turn doesn’t really do anything. You’ll find yourself in the same situation next turn, but this time without the Healbot. The correct play is putting down the Dr. Boom, because if you do manage to live, you can use him the next turn to clear your opponent’s board and then play the Healbot and keep yourself alive and no longer in danger of immediately dying. Look at the board and decide whether the safe play gives you any chance of winning before deciding to do it. Sometimes the risky play pays off, and it’s better than just floundering for a few turns before dying anyway!
