There has been a lot of thoughts on how Metallo became the well established super villain that he is today in the Superman books. However, only one person really gets the credit of making it possible, John Bryne. John has been a part of some of the most popular story lines of all time including: The Dark Phoenix story arc and Days of Future Past from the X-Men. He also had a memorable run doing the Fantastic Four which included the trial of Reed Richards after he saved the life of Galactus. In the 90's he also had a lengthy stint where he revitalized DC's flagship character, Superman. Byrne took a break from working at Marvel Comics and instead was hired by DC to do a retcon of the iconic superhero, Superman, which was needed after the Crisis of Infinite Earths storyline. This reworking of the superhero that is supposedly greater than any other in the eyes of the public got the media's attention and so articles were published everywhere including The New York Times. Byrne completely revamped Superman and himself stated that, "I'm taking Superman back to the basics...it's basically Siegel and Shuster's Superman meets the Fleischer Superman in 1986." Kal-El's mighty power was drastically reduced from his Silver Age version but he remained one of the mightiest beings in the DC Universe. Byrne changed the story so the Kents were still alive and often the support of Superman in his adult years. However Byrne got rid of Krypto, the Fortress of Solitude and the fact that Superman ever was Super-boy (which he regrets as it ruins the very idea of the Legion of Super-Heroes). John Byrne also made Clark Kent a man who actually fit in with society and was more of an ordinary human, or at least wished to be one, in his youth believing Earth to be his true home. This Superman made his debut in The Man of Steel a six issue miniseries describing his origins. Byrne did a lot of work on almost all of Superman's titles, especially in 1988 when it was The Last Son of Krypton's 50th anniversary. He continued this routine for two years but quit as he felt no support, especially since the line of Superman merchandise differed from the version he had remade for the comics.
With Superman relaunched, his first super villain encounter was not Lex Luthor but John Corben, better known as Metallo. Byrne established Metallo in his cyborg self with fake skin covering his robotic monker, and establshing that Corben had lost his humanity as time went on. This was reenforced more recently in the pages of Superman: Secret Origin with Metallo's retold origin, tying him in with General Lane and the rest of the Lane family and establishing just why he became obsessed with killing Superman all these years just as much as Lex Luthor or Darkseid have tried over the years. Hard to believe one simple story reestablished a villain and was the basis to who was today instead of a guy in a suit using kryptonite for a weapon.
Special thanks to Comicvine for the extra info.