1920s - Development of the game into a recognizable modern form, both on and off the ice. Off the ice, a transition from a low-money novelty enterprise to a large business with huge crowds spending real money, and the advent of mass media coverage. On the ice, the beginning of full-time professionalism, and the development of rules which set the template for a 5-man, forward-passing, speed-oriented game.
1930s - Survival. Between the Depression and imminent demands of WWII, a lowering of expectations for what the game could become as a business. Emergence of American markets as a dominant factor, on and off the ice. The apex of the first generation of true mass-market superstars, as compared to cult heroes of the prior generation.
1940s - Decline and rebirth. A half-decade of a league that was little more than a wartime entertainment gimmick, which had contracted to a minimal size and was on the verge of total irrelevance. And then, the dawning of a new golden age -- a new dynasty, and the emergence of a generation of superstars which would define the next era.
1950s - The "classic" NHL. Larger-than-life personalities, powerhouse dynasties, colorful underdogs, the beginnings of a TV broadcast culture that created a common experience.
1960s - An era of over-maturity, as the NHL held on to its small-league identity till after the common-sense expiration date. A league of extreme contrast between aging stars of the prior generation, and young talent which was too explosive to be denied a place at the table. And then... expansion, letting a sudden blast of fresh air into a stale league.
1970s - The wild west. Reckless expansion, the WHA, the rise of the Soviets, 70-goal scorers, bench-clearing brawls, goalie masks, extreme disparities between top and bottom talent. Anything and everything was possible. Most of it could only have happened in the 70s.
1980s - An era of dynasties. Firewagon hockey on the ice, a very conservative era off the ice. Consolidation and stabilization. Short careers. The record book being completely rewritten. Hockey encountering something of an identity crisis... is this a niche sport or is it part of a Big 4?
1990s - Big business. Another round of expansion, but not so reckless this time. Video games, ESPN, glow pucks, mega-star personalities. A disorienting geographic lurch. A revolution in the visual identity of the game. The peak of TV-friendliness... and then, just at the wrong moment... a pattern of catastrophic injuries, and a decline in the game.
2000s - The dark ages. A sport that only a hardcore fan could love. Faceless teams, wearing black alternate jerseys, playing 0-0 ties in half empty arenas. Championships won by attrition and 5% shooting. A lost season, and then a re-entry into a game that was hardly recognizable. But in the end, a stabilized league.
2010s - A comeback. A faster and more open game, a new generation of big-time personalities, the spectacle of 100K crowds, TV ratings on the rise, sleek new arenas, careful expansion, faux-vintage culture, analytics culture. The end of fighting and injury as essential elements. A league that's managed to get out of its own way, at least temporarily.