My writer's card will be revoked if I don't do a Prospect Retrospective for the retiring Mariano Rivera, so here it is.
Mariano Rivera was signed by the New York Yankees as a free agent out of Panama in February, 1990. He wasn't a big bonus guy, signing for only $3,000. He was athletic and had a loose arm, but didn't have a lot of pitching experience, was already 20 years old, and threw in the mid/upper 80s. Despite that, Yankees Latin American operations director and veteran scout Herb Raybourn liked Rivera and felt he was projectable.
His pro debut was very successful: he posted a miniscule 0.17 ERA in 52 innings (mostly in relief) in the Gulf Coast League, with a 58/7 K/BB ratio and just 17 hits allowed. He led the league in ERA. Despite that spectacular performance, he wasn't named as a top ten GCL prospect by Baseball America; he still didn't throw that hard, was older than ideal for the league at 20, and was considered more of a sleeper type than a hot property.
That began to change in 1991. Promoted to Low-A Greensboro in the South Atlantic League, he posted a 2.75 ERA with an excellent 123/36 K/BB ratio in 115 innings, with 103 hits allowed. His superior command got the attention of Yankees brass, including Buck Showalter. His ERA ranked 11th in the league and he still didn't make the top prospect list, but his velocity was picking up and he gave up just two homers on the season. He was a pitcher to watch and sabermetric types were aware of him.
Moved up to High-A Tampa for 1992, Rivera made 10 starts with outstanding performance (2.28 ERA, 42/5 K/BB in 59 innings, just 40 hits) but his season ended early with an elbow injury. Surgery was required, but it wasn't a Tommy John-style procedure. Still, his status entering 1993 was unclear. His record was excellent when healthy, but could he stay that way?
Rivera went back down to Greensboro in '93 once he finished rehab. He posted a nice 2.06 ERA in 39 innings, but did not show his previous command, posting a 32/15 K/BB; his walk rate was up and his slider and change-up needed work.
Fully healthy in 1994, he split the season between three levels, thriving in High-A (2.21 ERA in 37 innings) and Double-A (2.27 ERA in 63 innings) but getting hit hard after a late move to Triple-A (5.81 ERA in 34 innings). The important thing was his health: he had it back, but there were still questions about his secondary offerings, an issue which showed up statistically in a weak strikeout rate (just 61 whiffs in 97 innings in the high minors).
Baseball America had him ranked as the Number Nine Yankees prospect entering '95. He began with outstanding work in Triple-A (2.10 ERA, 30/3 K/BB in 30 innings) but struggled after moving up to the majors (5.51 ERA, 50/31 K/BB in 67 innings, 71 hits). At this point in his career, he was using a slider, change-up and splitter to go with his fastball. His velocity was increasing as the season progressed; he was in the mid-90s at times, which convinced the Yankees not to include him in rumored trades, despite his erratic overall performance.
This, of course, turned out to be an excellent decision.
Rivera moved to the bullpen full-time in 1996 and posted a 2.09 ERA in 107 innings as John Wetteland's main set-up man, with a spectacular 130/34 K/BB and just 73 hits allowed. He became the closer in '97 and, well, you know what happened then.
He was amazing and he stayed that way for 17 years. His key out-pitch for most of his career was a wicked cutter, which interestingly enough was not originally part of his arsenal; he didn't even throw it until 1997, and the cutter didn't become his go-to pitch until 1998.
Rivera is a certain Hall of Famer, of course, as ably pointed out by Rob Neyer, not even counting his ridiculously good post-season performance. His WAR value of 39.7 may not sound that good compared to some of the best starters in history, but WAR doesn't value relievers, even outstanding relievers, to the same extent it does starters.
Whether you agree with WAR's take on closers or not, Rivera's career 39.7 WAR is still an entire 12.1 points higher than his next-closest closer competitor, Goose Gossage (27.6), with Lee Smith (26.7), Billy Wagner (23.6), and Trevor Hoffman (23) coming up behind. Rivera's 652 career saves are 51 more than Hoffman, with Smith way back in third place at 478. Rivera for the Hall of Fame is a slam-dunk case; first ballot.
As a prospect, Rivera was notable for a tiny bonus and a later start to his career than most Latin American prospects, who typically sign at 16 or 17. He didn't throw hard at first and his secondary pitches needed development, but he threw strikes and put up excellent K/BB ratios when healthy. He was the diamond-in-the-rough that every scout dreams about.