#2 His style of management
Sir Alex Ferguson was in every word the 'manager' at Manchester United, having that old fashioned model whereby every decision revolved around him from training methods, fitness regimes, to decisions on arriving players.
While the modern day coach still has the final say on most of these matters, their overbearing influence at clubs have been removed from some other mundane things, leaving the coach to focus on other task specific roles such as tactics and motivation of the players.
When Moyes arrived at Manchester United, he was said to be shocked at how outdated some of the club's methods were, considering the size of United, and considerable time was spent trying to modernise the club's methods, including scouting reports.
Ferguson was a great manager, and deserves all the accolades accorded him, but the greatness and unyielding power afforded him at the club was detrimental to United's fortunes.