There are many different ways the league has evolved over the years. Some of it just deals with numbers while other evolutions have come about for physical reasons, such as equipment. Rather than chart the progression of the league in a chronological fashion, the evolution of the league can be best described through several different topics.
The Teams in the League
When the newest franchise, based in Las Vegas, officially joins the league for the 2017-18 season, there will be 31 teams. That is a far cry from when the Great Depression and World War II caused the NHL to shrink to the original six teams located in New York, Toronto, Boston, Chicago, Detroit, and Montreal). It stayed that way until 1967 when six new teams were added.
Since then, the NHL has grown at a much faster rate. By 1979, 21 teams were in the league. From 1991-2000, nine more teams were added and that was where things have stood until the recent inclusion of Las Vegas. For a more detailed look as to which franchise entered the league, try this page.
Evolving Equipment
Back in the older days, not even goalies wore hockey masks. These days, goalies were masks, for incorporated in 1959, and later bigger pads to keep scoring down. A chronological list of how goalie equipment has evolved can be found on this site.
Players also wore better gear as materials made such gear lighter and more efficient. The skates are much sleeker, creating greater speed on the ice.
The Athletes Themselves
Modern NHL players would are bigger than their counterparts from half a century ago. The average player grew two inches and weighs 20 pounds according to a Ken Campbell article in The Hockey News in 2015. Many regularly blast shots that go whistling toward the net at over 100 miles per hour. Despite that, hockey remains skill-oriented. So the players are still very skilled, just much bigger.
Changing Rules
Because of the growth, there are now more rules in an effort to keep the league competitive. There is a salary cap in place to ensure that all teams have a chance to compete and the league won’t be dominated by bigger markets. Rules changes such as larger pads and the “crease rule” ensure that players are better protected despite being larger and faster than ever. There is a sudden-death overtime and a shootout even after that. There are video replays to verify goals now. There are puck trackers to help a TV audience follow the action.