Cesar Cedeno - would remember looking at my baseball cards 83-84 ish when I was young and being amazed how good he once was (before my time) - he became pretty average by the 80's.
Looking at what he had accomplished by the age of 23, he looked like a really good chance to be a Hall of Famer that sort of fizzled. Not that 2,000 career hits is a bad career, but it was disappointing based on how he started.