One of the biggest mistakes the Leafs made was trading away Danny Markov. He was a tank. He was a very good defensive defenseman - tough, but clean. He was probably the best penalty killer after Yushkevich. From what I recall (I don't think the stats are available anymore), he was a tremendous shot blocker too. The very next season, in Phoenix, he averaged close to 23 minutes per game on a 95 point team.
In exchange, the Leafs got Robert Reichel (a soft, inconsistent centre - who vanished in the playoffs and never really found a dedicated role on the team), Travis Green (a good penalty killer, but he could have been replaced), and Craig Mills (never played a game for Toronto).
Granted, I'm not sure if having Markov would have gotten the Leafs past Carolina in 2002 (and even if that happened, they would have been obliterated by Detroit). But Green and Reichel combined for zero points against Carolina.
A blueline with Yushkevich/Kaberle, McCabe/Markov, and Berg/Lumme looks a lot better. Hoglund/Sundin/Renberg would remain the top line, and Roberts/Tucker/Mogilny would have been a very good second line. Valk/McCauley/Domi was a solid fourth line (two PK'ers and a goon). The third line would be weakened without Green, but it could have been built around Corson. I think this version of the Leafs would have done better (that way, McCabe and his boneheaded players wouldn't have got 25 minutes a night, and Berg wouldn't have been forced into the #4 position).
(The biggest what if, when it comes to defensemen, is if the Leafs got Robert Svehla earlier, and/or if he stayed in 2004. He was only there for one year, but he probably had a better season than any other blueline during the Joseph/Belfour era. He was a beast. He matched Yushkevich in checking and defensive ability, but was a surprisingly good playmaker. Maybe if he was around, management wouldn't have turned the locked room into a geriatric convention in 2004).