Here is my friend Waskir's top 10 for arena misplays. He's an EU legend but mainly play a lot of arena with a winrate of ~75%. I hope you like it, have a good laugh or that this post even helps you to improve your game. (Reason I'm posting is the fact that he's having trouble posting it himself)
No. 1 Missing Lethal: Sometimes you get so occupied keeping the board clear that you just continue clearing the board instead of just killing your opponent. Sometimes this outright costs you the game since it gives your opponent a potential out. This mistake even happens to the best players so don't forget to constantly check for lethal. On a higher level of play you can miss lethal one or even two turns before actually killing your opponent. A simple example is to have a Fireball in hand and missing to put your opponent to 7.
No. 2 Forgetting to attack: This mistake is so silly.. yet it happens to the best of us. Plan your turn --> Execute your turn --> Take your time to check your play --> Hit the "next turn" button.
No. 3 Playing for too much value: This is something I have seen plenty of times and I consider it to be one of the most frequent and most terrible mistakes of lower level arena players. The classic example is to just use the hero power or playing an inferiour minion instead of lobbing out that Mind Control Tech on turn 3, since it's battlecry is useless. A really bad player even continues to wait the following turns hoping for the opponent to overcommit taking a lot of damage in the meantime.The correct play is to just play that Mind Control Tech if you don't have a better alternative. By not playing that MC-tech turn 3 you are essentially giving away board control as well as the control over the game for a 25% (or lower) chance to get their best minion on turn x, which might still lead to a loss if your opponent has an answer.
No. 4 Getting greedy: You got offered a Ragnaros, an Ysera, 3 Boulderfist Ogres, 2 Ancient of Lores, 2 ancients of war, 2 iron bark protectors, and you took them all? You should probably be extremely happy if you ever get more than 3 wins! Having all these amazing cards is great. The problem is the probability for having 2 or even more dead cards until turn 6 is so high that your precious ragnaros will never see daylight since you are already dead.
No. 5 Playing like in constructed decks: A lot of plays that might be great for constructed are highly questionable in the arena. I have seen a lot of players using Wrath to cycle through their deck instead of just using their hero power while being on perfect health. This might be an ok play when you are playing a ramp druid knowing what deck you are playing against. First, in arena you most likely don't have a ramp druid deck. Second, you have no idea what you are up against. Usually you will regret your misplay just the following turn, not having that extra 3 dmg.
No. 6 Drafting for constructed decks: A lot of players seem to try drafting a deck that is as close as possible to a constructed deck they know. Occasionally they end up with decks that just crush through the arena like a beast. However, most of the time they will end up with unfinished "half-combos" since there is no guarantee to get all the required cards. For example Shadowstep is a terrible pick unless you already have a god-tier card to combo it with.
No. 7 Bad Mulligan: So you are going first and you got three 4-drops in your starting hand? Well that's bad luck. But if your deck is rather expensive you might want to keep at least one of those 4 drops since getting an even more expensive hand happens more often than you might think. One could write a whole chapter about how to mulligan correctly but the main remedy to this mistake is to get familiar with your arena deck and it's mana curve specifically before even starting the first game.
No. 8 Having inflated expectations of your arena deck: This might seem a bit odd but I have seen streamers falling prone to this, and I have catched myself thinking: "This deck is awesome, I'll get to 12 wins with this!". All of a sudden you lose 2 early games and get tilted, leading to a misplay causing an unnecessary third loss. Never forget that RNG plays a big role in hearthstone, especially in the short run. If your new forged "imba-deck" loses due to abysmal draws or to opponents with even more ridicolous decks there is nothing you can do about it. Suck it up and play your best. In the long run you WILL perform according to your drafting and playing skills.
No. 9 Killing yourself: You got a bunch of weapons? Awesome! You can trade superefficiently almost every turn? Awesome! You get a Fireball out of the blue and lost the game? ... Oooops! Never forget to keep an eye on your lifetotal. If you are dropping low you need to settle for less efficient trades or you'll get maged!.
No. 10 Drafting too many situational cards: In arena it is easy to pick up too many cards that are situational (e.g. Betrayal) or cards that rely on a specific combo (e.g. Timber Wolf). Even if these cards were the best pick at the time and their potential value is enormous you want to keep them to a minimum. Otherwise you'll end up with a hand full of situational cards not having any play for a turn since the requirements are not met. This is one of the main reasons blunt cards like Chillwind Yeti or Boulderfist Ogre are so strong in arena. It's rarely wrong to just play them.