#1 Attitude and tactics
United players do not possess the consistent winning mindset that is associated with playing for the club. Ferguson used to inspire fire in the hearts of the players. They would dedicate themselves to winning even their backs were against the wall.
Since he left, the team's attitude and mentality have never been the same. This is reflected in the style of play. On one end, United would struggle to break down minor opposition on home turf. On the other end, they could visit the Emirates of the Etihad and secure comeback wins.
This inconsistency has greatly affected the team with calls for more attacking displays. Fans have been accustomed to not to know what to expect from the team. United need to show more bravery in order to return the fear factor. At the moment, most teams regardless of their pedigree have the belief that they have a chance when it is Manchester United on the other end.