Michael would have had more time to learn from his father, and would not have been distracted by the ever whiny Kay.
Appollonia was a good Catholic girl who would never dream of having an abortion.

Also, the Corleones would have had closer ties to Sicily because Michael and the family would pack up every summer for a month at their Villa. This would have brought him even closer to Dontomassino, his other mentor, and thus would have made him an even sttronger Don.

Michale's kids would have turnd out much better, and Apollonia would never have allowed Mary to date her cousin.

Most importantly, after the moved to Vegas they would have served peppers and salsice at the First Communion Party, and Michael would still be drinking red wine and not champagne cocktails.