While we have talked about the character flaws of Vito and Michael, perhaps we have overlooked one thing. Vito saw first hand, as a child what happened when the Mafia was crossed. He witnessed his mother die, and was shipped to the U.S. where he had to learn a new language and a new culture. When he got fired from the grocery because Fanucci forced Genco Sr. to hire his nephew, Vito understood he was going nowhere in the world unless he chose the life he lived. He was already mixed up with Clemenza and Tessio selling stolen clothing, stealing rugs, etc., and when confronted by Fanucci he understood he had to take Fanucci out and run his own "business." Later in his life he told Michael he had no regrets because he did not want to be some kind of puppet manipulated bu others. Throughout his life Vito probably believed this rationale, and that allowed him to have a good marriage, to love his children, to maintain friendships, and to show respect for others.
Michael was totally different. He grew up affluent. HE went to an Ivy League school, and while he was certainly happy to live in that lifestyle, he was also a rebel who joined the army and took pleasure in telling his father he would never work for him, nor be a man like him.

Then after Vito was shot and Michael saw that it was necessary for him to become involved in the family busines, he did so with a vengence. Its kind of like someone who converts from one religion to another...people like that are probably a bit more zealous than those born into it.

So for Michael being the Don had nothing to do with trading favors, showing respect for others, or even loving his family (clearly the only reason he took his kids away from Kay had nothing to do with his caring about them, but instead it was a way to get back at her for "leaving" him).

A good example of the difference is een in the treatment of Johnnie Fontaine. To Vito he was a beloved godson who he could help, but who he could also reprimand, and give advice. To Michael Johnny was just a cash cow. In GF III Michael doesn't even give Johnny the courtesy of litening to his song, but instead humiliates him by making a reference to Tony Bennett and leaving the room. Vito would have never done a thing like that because such rudeness, to Vito, was an infamnia.