Ferguson had an almost dream of a career at the Old Trafford. I have made an attempt to find some of the key reasons why he succeeded.
1. Man management skills:
He knew his team very well. He knew exactly what he could expect from every player and how he could extract the best from even temperamental players. Look at players like Roy Keane and Eric Cantona- they had explosive personalities but they were the vital cog in the wheel of United during their stay.
Anderson too was at his best during his tenure but faded under Moyes and now under van Gall- shows a lot about how Fergie knew how to get the best from him.

2. Shrewd Business skills:
He was well aware when he had to sell a player and buy a player. Players like Stam, Ronaldo were sold unexpectedly. But the team sill survived to win more trophies each year. And he purchased some gems in the market like Ronaldo, Ferdinand, Vidic, Rooney, Carrick, De Gea, etc.
And what do I say about the transfer of van Persie? "Arse end" Wenger!

3. A tactical mastermind:
He knack for getting the substitutions right each and every time was a priceless asset. From getting Solksjaer on for the UCL finals to getting Macheda on for the AVL game, he has managed to put on match winners on to the pitch on many occasions.
Plus the term "Fergie Time" played a lot on the oppositions mind whenever he played with his team.

4. An eye for talent:
This man had an eye for talent. The list of talented footballers he and his team of scouts had discovered is almost endless. Scholes, Giggs, Neville Brothers, Becks, Butt, Ronaldo, Da Silva twins, Pique, and I can go on with that list.
The best thing about Fergie was that he had a tremendous amount of faith in the youth. He never hesitated in putting a youth player on the field. That desire made United as one of the most coveted clubs when it comes to playing homegrown players.

5. Knighting the Leader:
Fergie always had his army spearheaded by a powerful on pitch leader. Bruce, Vidic, Ferdinand and even in Rooney he found some of the perfect guys to lead his able warriors.