After finishing dead last in 77-78, Lou Nanne began the task of rebuilding the Minnesota North Stars. Helped initially by the merger with Cleveland, combined with some shrewd draft selections, the results were impressive; A jump from 45 to 68 points in 78-79, then to 88 points in 79-80 (along with the huge upset over Montreal in the playoffs). Then in 80-81 they made it to the Stanley Cup Final. They were easily beaten by the Islanders in that final, but the future looked very bright.
http://www.hockey-reference.com/teams/MNS/1981.html
Don Beaupre (age 19)
Dino Ciccarelli (age 20)
Tom McCarthy (age 20)
Neal Broten (age 21)
Craig Hartsburg (age 21)
Curt Giles (age 22)
Steve Payne (age 22)
Bobby Smith (age 22)
Brad Maxwell (age 23)
Gordie Roberts (age 23)
I can't think of another Stanley Cup finalist that had a core of good players that were this young. And Brian Bellows would be drafted the following year. With their youth, along with the playoff experience of the past two seasons, it looked like they would become one of the dominant teams of the 80s.
Instead, things didn't quite work out. Despite being placed in the league's weakest division, they only made it past the second round once in the rest of the decade; and by 87-88 they were exactly where they were 10 years earlier: dead last overall.
What went wrong with this team?
As someone who has spent the last few years reading, researching, and writing about the Minnesota North Stars from this era I feel I might be able to provide some insight into this.
Indeed, the North Stars were beginning to look like quite the contender by the early 1980s with their core of young players and a deep postseason run in 1981. However, as the 80s gradually wore on Minnesota seemingly got lost in the shuffle of young talented teams, taking a back seat to the likes of the Edmonton Oilers (who made short work of them in the 1984 Campbell Conference Finals) and Calgary Flames.
I think part of the problem with the North Stars was based on what Pierre Page said about the team upon taking over as coach when they hit rock bottom in 1988. He claimed that Minnesota had become "mesmerized by talent." In other words the North Stars were a very skilled and talented group of young players, but they seemingly lacked the veteran leadership, work ethic, and heart to play disciplined, defensive hockey for any sustained period of time. Keep in mind that while the Norris Division was weak, it was also one of the toughest divisions back then. A skilled, talented team with questionable heart, work ethic, and even toughness in a division like that would not be too successful and I think that's why the Stars were not as successful as expected.
Now onto the players you mentioned.
Don Beaupre (age 19) - Had an excellent rookie year, but every year after that with the North Stars paled in comparison. He struggled to maintain consistency for much of his tenure with Minnesota and found himself sharing playing time with the backups like Gilles Meloche and Kari Takko. Beaupre was dealt to Washington for Claudio Scremin (who?) in November 1988. He would resurrect his career with the Capitals but before that Beaupre was, in many ways, a one-season wonder.
Dino Ciccarelli (age 20)- His career with the North Stars started off with a bang and he was one of the team's most prolific goal scorers during those early heady years. However, Ciccarelli's off-ice antics began to overshadow his on-ice performance. Eventually, it had an effect on the team. Many people questioned his leadership too and felt that his behaviour set a bad example for what was a young and impressionable North Stars roster (the same accusations were thrown at Brian Bellows later on). Page didn't see eye-to-eye with him and in March 1989 he was traded to Washington for Mike Gartner whom the coach felt was a better leader.
Tom McCarthy (age 20) - Had an extremely promising career in his early days, but was stricken with Bells Palsy 25 games into the 1985-86 season. This effectively ended his NHL career at the age of 26.
Neal Broten (age 21) - Broten was arguably one of the best-playmakers in the game at the time. After being the first American player to score over 100 points, however, his career went off the rails with a series of injuries that wreaked havoc on him for the next two years. It's no surprise his decline mirrored that of the North Stars. Nevertheless, he was able to get his NHL carrer back on track in the late-1980s/early-1990s. Being a little older and wiser probably helped him in the leadership department too.
Craig Hartsburg (age 21) - Another player whose career was destroyed by injuries and ended too soon. Hartsburg was fast becoming one of the NHL's best blueliners when a laundry list of injuries derailed his career. By 1988 he was a shell of his former self and he retired in January 1989.
Curt Giles (age 22) - Underrated as ever. Giles was one of the North Stars' best leaders through thick and thin. Still, he was not at the elite level of the other players you had listed here. Size was also an issue. He was good and excelled beyond expectations, but he was not great.
Steve Payne (age 22) - Like McCarthy, his career ended way too early. Multiple cervical spine injuries.
Bobby Smith (age 22) - Any North Stars fan will tell you the beginning of the team's fall from potential greatness started with Bobby Smith's trade to Montreal in 1983. It is widely considered one of the worst trades in the franchise's history and it left a hole at center that was not filled until Mike Modano came along. Those post-Bobby Smith North Stars teams always lacked size down the middle. Thankfully, the team righted a wrong and brought him back in 1990. Mind you, he was past his prime at this point, but he was still invaluable to their '91 Cup Run.
Brad Maxwell (age 23) - Had a promising early NHL career, which peaked in 1983-84 before inexplicably declining fast. Bounced around a lot in his latter days before retiring in 1987.
Gordie Roberts (age 23) - One of the guys that a rebuilding Minnesota team traded away prematurely. Roberts was one of the few who went on to have a long and successful career.
