Hearthstone sucks. The thing they did wrong, from the beginning, with this game was to make the Arena a pay to play format. It is the best version of the game, and there are some simple things they could do to make it better.
1. Barrier of entry. You have to pay in money or gold to participate. You have to perform to a certain level just to break even or do better than your investment. This in combination of the randomness of what card you get to pick causes a lot of people not to even bother.
2. It takes too long. You can spend 10-15 minutes drafting a deck, then over an hour playing your run. The 10-win, 3-loss game format is too many.
3. Random Hero choice. This is them admitting that they cannot get balancing correct to save their lives. The only reason they randomize and make you choose one of 3 hero's is because if they let you pick, everyone would pick the consensus 1-2 best heroes. Making for a dull format. Why not have a running pick % with the lesser picked heroes getting an extra chance to draft higher quality (epic/legendary) cards.
4. Why is there no ranking system? Arena could/should have a ranking system just like the constructed format. Give monthly rewards.
5. Give more free-rolls. If you are going to continue to make it pay-to-pay, at least give away a weekly free-roll to entice more people to try it.