Good article in Harvard Business Review (http://hbr.org/2013/10/fergusons-formula/ar/1) which identifies a number of key traits that underlined his success.
When I read them, I felt the headlines summaries of his qualities I did not feel they went deep enough. So based on what the article said, these are what I believe were his real qualities of leadership:
1) Focus on talent – acquiring, nurturing and holding onto them
2) Courage to make the right decisions
3) Set clear expectations of quality
4) Focus on instilling values more than just building capabilities
5) Respect (for self from others and respect for others)
6) Self belief (so not distracted and able to make incisive decisions)
7) Great sensory acuity – so really ‘see’ people and so can respond appropriately
8) Flexibility and openness to change
I am not a great football follower but from what I have heard no player from his period has become a great Manager themselves. I am sure his public persona was different from what he was like one-to-one and I am sure his passion and care for his club and people were in many ways the key drivers of his success.