What made Mariano Rivera great? Major League Baseball teams have an abundance of pitchers that throw 95 to 100 mph. What separated him was his command, movement and mental toughness in game situations.
Rivera had great command of a rising fastball when he first came to the big leagues and later developed the pitch he is known for—the cut fast ball—when hitters starting laying off the riser because it was rising out off the strike zone. The brilliance of Rivera and Greg Maddux, for example, is that they knew when the ball left their hand exactly how much the ball would break. This ability is obviously superior command and makes pitching to hitters' weaknesses easier. Most pitchers HOPE that happens, Rivera and Maddux almost always knew.
Movement takes hitters down a lot easier than speed. Listening to most big league hitters, they can catch up to any pitcher's fastball. But they hate to see late action, such as a jump or hop in the ball that makes it rise, diving sinkers, Rivera's late-breaking cutter or Maddux's comeback fastball. The hitters are at pitchers' mercy when they throw those types of pitches and they know that!
Lastly, maintaining poise and mental toughness in crucial situations. All the great pitchers have those attributes, but we have seen pitchers with better stuff than Rivera and Maddux fail because they can't handle the in-game pressure or their mechanics are not game sound, which means they rely on power without technique.
Rivera and Maddux had great and simple mechanics which allowed them to repeat their deliveries and concentrate on getting hitters out without worrying about mechanics.