Biased answer: Everything
Attempt at Objective Answer:
The style of play which the manager Arsene Wenger is largely responsible for. Arsenal play a quick short passing game which is great to watch.
The manager for playing young players and unearthing new talent and producing consistent results in the league and in Europe
The running of the club. Not in debt like other big clubs and are self sustaining. The Board is run by people who genuinely love the club.