re: What are some notable baseball careers that started off HOF worthy only to derail badly.

Eric Davis. Was supposed to be the next Willie Mays. Injuries took him down.
In his first full year of professional baseball, Davis pilfered 40 bases in 62 games.[3]

When Davis first appeared in the major leagues in 1984, his physical talents gave him the potential to be one of the most exciting players in the game. He was a rare five-tool player with home run power as well as sheer speed on the base paths. He made a habit of robbing home runs and elicited comparisons to Willie Mays.

Davis began to excel in 1986, batting .277, swatting 27 homers, and stealing 80 bases. He and Rickey Henderson remain the only players in major league history to be members of the "20/80 club".[5] In a 162-game span—June 11, 1986 to July 4, 1987—he made 659 plate appearance and batted .308/.406/.622 with 47 homers, 149 runs, 123 RBI, and 98 stolen bases. (He was caught stealing just 12 times.)[3]

Davis continued to build on his success in 1987. On Opening Day, he went 3–for–3 with a home run, a stolen base, and two walks. Through the first 10 games, he was batting .526 with 4 home runs and 8 stolen bases. On May 1, 1987, he hit 2 home runs, including a grand slam. Two days later, he hit another three home runs—one each to left, center, and right field—including a grand slam, and a stolen base. He hit another grand slam that month, making him the first player in history to hit three in one month. During an eventful play in the late innings at Wrigley Field on September 4, Davis crashed into the outfield brick wall as he caught a deep fly ball; he laid on the ground several moments and was slowed afterward.[3]

Davis finished the 1987 season with a .293 average, 37 homers, and 50 steals. He became the first player in history to hit 30 homers and steal 50 bases in a season, despite playing in only 129 games.[6] Prior to 1987, just six players had achieved the 30–30 club. That season, three others—including Strawberry, Joe Carter, and Howard Johnson—joined Davis in the 30–30 club.[3] He led the league in power-speed number (42.53) with a mark that is the third-highest single season mark ever.[7]

From 1986 to 1990, Davis averaged 30 home runs and 40 stolen bases. During this time, he was one of the game's most exciting players and a very visible superstar player. He drew some MVP support every year from 1986 to 1990, finishing in the top 15 in the voting every year. From 1986 to 1989, he also finished in the NL's top 10 in home runs, slugging percentage, and OPS each year. While he had some other good seasons later in his career, injuries prevented him from reaching this type of peak again. In 1990, with a solid team around him, Davis was a key player in Cincinnati's "wire-to-wire" championship season.