I came back to Hearthstone after a ~2 month break and noticed I did significantly worse in arena. I used to get 5+ wins pretty consistently, now I lose a lot of runs at 3-4 wins. The average skill level has definitely gone up, or maybe I haven't adapted to the new arena meta (which changes every set.)
Ideally, you want to find ways to trade efficiently on your turn, and find ways that prevent your opponent from trading efficiently on their turn. Sometimes this means leaving a minion at 5+ health against Paladin because the most common removal caps at 4 damage from Truesilver. Sometimes it means avoiding having a 1 health minion against Mage, or trading such that you have multiple 1 health minions because they can only kill one of them.
IMO, the hard part of Hearthstone is deciding what cards to play around and which ones to not.