I think both of them come into play. Here are examples of both
Bad plays in Arena-
Not playing against class specific removals/AOE. Eg. Flooding board against mage on a turn 7. Keeping 2 drops on board against Paladin/Priest on turn 4/5.
Trading minions inefficiently- You have to know when to go face or when to trade with a minion. There are times when you know your opponent will definitely trade into your minion so you can go face with that minion (you basically gave it windfury if you opponent attacks with his minion into it next) This isn't always the correct play however.
The main and most important point here is to know what kind of removal/how the class can deal with your played minions. Eg. playing 1 health minions on turn 2 against classes like Priest/Hunter is mostly safe because they don't have many ways to ping it.
Bad draft choices -
Picking cards which depend on other cards to be good. Eg. Picking a Stoneskin Gargoyle if you have a Blessing of Kings. Its good when it works but you mostly just get 1 card of each out of 30 and you can't always expect them to be in your hand. Play a Gargoyle w/out a buff and you just gave your opponents minion a free trade + a card.
Picking situational cards - These cards may require you to have some sort of board/have your opponent play some kind of minions. These are also good when they work but sometimes they end up being dead cards in your hand for a while. These cards shouldn't be avoided but you shouldn't have too many of them. Eg. Soul of the forest
Drafting too much early game- This was one of my biggest problem before I became good at arena. You should have only enough early game to carry you to the mid/late game. (This isn't the case always though you might get a rushy draft but it happens rarely) A 5+ mana minion can deal with at least 2-3 early game minion. So your opponent gains a massive card advantage by this.
The main point here is no matter how good your deck is you might lose to some BS in arena. Don't let that make you think you played badly. Also class here powers help a lot. Mage/Paladin have the best hero power followed by Shaman, Druid, Warlock Rogue, Priest, Warrior.
This is all my opinion though it doesn't have to be completely true. Feel free to take these into account to help you figure out what you're doing wrong in Arena.