#1 His coaching style and tactics

Unai Emery has been described as a rather meticulous tactician. The new breath of fresh air in regards to tactics should help Arsenal improve over time, as players became complacent and predictable after repetition under Wenger's management.
Teams were able to counter their style and the Gunners' dominance was soon at its end, where their nice passing play was forgotten as it didn't always result in three points on a matchday.
Emery loves playing with possession of the ball and if his players don't have that, he demands they are very intensive with their pressing style. This will be something new to most players, who will hopefully manage to take it on board and collectively improve, knowing what their individual responsibilities are and sticking to them as a team.