This is a brilliant piece on many levels.
By talking to so many sources (I counted 10, not counting Rivera), from all different aspects of baseball, it shows just how respected/feared Rivera and his "cutter" are.
The most impressive part to me is that regardless of what type of baseball fan you are, Olney is able to explain how dominant Rivera is. He's got stats to back up the dominance, anecdotes from All-Star across the league and even a "Sports Science" type of breakdown. All of these viewpoints are on display and explained in a way that all fans can understand. The stats aren't too complex for those who are sabermetrically disinclined, the stories give you an "inside baseball" feel without making the reader feel like an outsider and while the technical breakdown is good, you don't feel like you had to have pashed Physics to understand.
The multiple angles also makes it a surprisingly quick read and not make it feel like Olney is banging you over the head with the argument, even though it is nearly 2,000 words long.