#3 Pressing and energy
Led by the presence of Torreira, Arsenal were pressing Liverpool from the start and were able to get in Liverpool's faces and manage to deny them space in this area. The midfield battle was primary in winning this game, and Arsenal arguably did this via strong tactical planning from Emery.
Fabinho was a top signing in the summer for Liverpool, but he was matched for energy by Torreira, along with Xhaka and Ozil.
Milner is an underrated player somewhat, and despite his goal didn't really challenge in the midfield that much. It was all down to Emery's game plan, which sought to match Liverpool's energy and fight. And this is exactly what happened.
Another point was the early direct balls to Aubameyang, which capitalised on Liverpool's backline, and created some early chances for Arsenal in the first half. It is Emery's meticulous planning that is showing the improvements required, especially compared to the latter part of Wenger's tenure.