Bias
I think that Len Bias is a big mystery. Not his talent on the floor, which was awesome, but the way that he lived his life. He was portrayed as an All-American Christian, but he did cocaine. Some people said that he went out with both good girls and bad girls. Who really knew what this guy was actually like?
But, let's say that that night was an abberation, and that he lived, the history of the Boston Celtics would have been different, obviously. In 1986-87, he would have been a big help with Bird and McHale. He would have taken away minutes from them, which would have preserved them for the playoffs. McHale wouldn't have gotten the stress fracture in his foot that he did that season. They would have beaten the Lakers in the Finals in six that year.
The next year, they get back to the Finals after beating the Pistons in seven, but the Lakers beat them this time. In 89, the Pistons finally dethrone them in six games, and dethrone the Lakers, as in OTL. In 90, the Pistons repeat after beating the Bulls, Celtics, and Blazers.
In 91, people forget that the Celtics opened the season the first few months with the best record in basketball. Bob Ryan of the Boston Globe said that if they had the playoffs in Dec. 1990, they would have won. With Bias, they finish with the best record in basketball. Bird probably wouldn't have missed much time with a back injury, and they go back to the Finals after beating the Bulls in the East Finals and win it all over the Lakers.
In 92, the Celtics would have made the Conference Finals against the Bulls, but I think the Bulls finally would have broken through. But the Blazers, with the Finals experience, would have beaten them in six games. They should have at least taken them to seven games that year anyway. By 93, though, the Bulls would have won their first championship over the Suns.