Lack of title-winning pedigree
With the exception of Mesut Ozil, Arsenal don't have of member of their squad who is used to winning titles, or one who's experienced in what it takes to maintain a challenge.
I'm not saying that a team who hans't won something before can't go on to win things but it is a definite advantage that their nearest rivals will have over them.
Chelsea, Manchester City and United all have personnel who are used to winning trophies and when it comes to sitting at the summit of the Premier League come the latter stages of the season, Arsenal's current crop will be in unfamiliar territory.
That being said, Wenger knows how install confidence in his players, and the fearless attitude the likes of Ramsey and Wilshere possess may serve as a boost that the other title-hopefuls don't have.