#3 The sustained level of performance late into his career
It's hard to believe that Michael Schumacher won the bulk of his titles in his 30s. Although a testament to the level of physical conditioning he had done for himself, Schumacher was as competitive in an F1 car in the 30s as he was in his 20s. The German became the gold standard of racing in F1 in 1994 after Ayrton Senna's tragic death.
After winning his first title, however, Schumacher never let go of that crown until his first retirement in 2006. At the time, he was still battling for the title at the age of 37 against a very young Fernando Alonso who was in his mid-20s.
Schumacher showed an extraordinary level of longevity in terms of performance and success in his car, something that was taken as a benchmark by drivers like Alonso and Lewis Hamilton who are now in their late 30s and early 40s.
#2 The entire team revolved around him
What made Michael Schumacher brilliant was his ability to not only be a leader but a manager as well. In the documentary, "Schumacher," his wife Corrina talks about how he remembered key details of each and every member of the team. He knew the names of all the mechanics, their birthdays, their children's names, and whatnot. He made it a point to thank them after every race irrespective of the result.
What this did was it brought the entire team together in support of him. They worked extra hard because they knew he was going to extract whatever was possible in that car.
#1 His ability to build a championship-winning team
Michael Schumacher is one of the few drivers in the history of the sport that was responsible for the rise to prominence of three teams. First, it was the turnaround achieved at Benetton, where the team transformed itself from being a midfield contender to a two-time championship-winning outfit.
After Benetton, Schumacher went with Ferrari on an unprecedented run of 5 consecutive driver titles. That marked an era that is still considered legendary in the history of the sport. The lesser spoken of contribution by Schumacher was at Mercedes. The German driver came out of retirement and drove for the team for three years. During that time, in conjunction with Ross Brawn, Schumacher set up the foundations of Mercedes, a team that went on a charge in 2014 and has not lost the title since.
Schumacher had the uncanny ability to know how to develop a championship-winning team, something that he was admirably able to put to good use.