#1 Not laying down a unique playing pattern
Right from his appointment in 1986 till his retirement in 2013, Sir Alex Ferguson dominated English football in a manner not seen before, but contrary to what many would believe, his dominance was not built on having much tactical prowess, but rather on excellent man management, and the ability to extract the best from his players on a weekly basis.
United were exciting to watch under Fergie no doubt, but the club only started to show tactical variation in the 2000's upon the appointment of Carlos Queiroz as assistant manager.
It is no hidden secret that English managers are tactically behind their European counterparts, and it is largely for this reason that United and Fergie bossed their way around the Premier League in the 1990's.
Their dominance began to quaver when a sophisticated Arsene Wenger with a tactically sound mind arrived the league, and it further reduced when Jose Mourinho came to Chelsea in 2004, with Ferguson struggling badly against the Portuguese (he has a very poor record against Mourinho).
Proof of Fergie's tactical shortcomings are perhaps best exemplified by his struggles in the Champions League, as United failed to translate their domestic dominance in the 1990s onto the continent (bar their 1999 triumph) until Queiroz came onto the scene in the latter part of the first decade of the millennium.
Perhaps the best sign of a great coach is the number of players who go on to achieve successful coaching careers after him, and it is in this regard that Johan Cruyff could be considered the greatest ever (just look at Pep Guardiola, Luis Enrique, and Ronald Koeman), and here also Fergie is fatally indicted, as none of his charges have gone on to achieve great feats as managers, with Roy Keane being the latest in a line which also includes Gary Neville, Mark Hughes, and Steve Bruce among others.
Most clubs have a unique playing pattern which they follow. Cruyff for example laid the foundation for possession based football at Barca & Ajax, and any incoming coach knows the template to follow, same with Wenger at Arsenal, and as such an ideal manager is already in mind before any new appointment is made.
Ferguson failed to lay down a distinct playing pattern at United, which is rather surprising considering how long he spent at the helm, and it is this lack of playing identity which means the club struggles in making appointments, as there is no template to work with, and this is something which could have been averted had Ferguson been more tactically astute.