For the intents and purposes of this article, scoring efficiency will be defined by "Points Per 100 Possessions" (hereafter referred to as "PPP"). This nifty statistic can be found at basketball-reference.com. The rule changes I list are courtesy of nba.com.
Our journey begins in 1974, as this is the year turnovers, blocks, steals, and off/def rebounds become tracked statistics. "Possessions" cannot be tracked without knowing how many were thrown away (differentiating between offensive and defensive rebounds also helps).
SIGNIFICANT RULE CHANGES
1977: This isn't a rule change, but is significant anyway. The NBA completed it's merger with the ABA, and expanded the league's talent pool. The league would soon crack a 1:1 point possession ratio and never look back.
1979: Clarification added to prohibit hand-checking through “rigid enforcement” of rule allowing a defensive player to retain contact with his opponent so long as he does not impede his opponent’s progress.
1979 was the beginning of the end for handchecking, and teams jumped 3 PPP with the decrease in physical play.
MOVING BACKWARD TO THE PAST
The NBA was not very efficient back in the 1970S, and this data doesn't lend much credence to the bloated numbers of the 50s and 60s.
Wilt Chamberlain famously averaged 50.4 points back in 1962, but took 39.5 FGA and 17.0 FTA per game to get there. We don't know how many turnovers he averaged, but chances are, it was A LOT (probably at least 5 per game). It is highly unlikely that Wilt scored at a rate higher than 1 PPP... and that's not exactly a ringing endorsement of his teammates or the rest of the league.
Because turnovers were not tracked, we can't know how many possessions the Philadelphia Warriors had. What we do know is Philly led the league in scoring (125.4) AND gave up the most points per game (122.7) in the league.
It is VERY safe to say Wilt's numbers were inflated by the pace his team played, and the pace his league played in. The Warriors averaged 111 FGA and 40 FTA per game. Today's Warriors (the league's highest tempo team) average only 85 FGA and 27 FTA, while also averaging 15.7 turnovers per game. Using that same turnover rate, the 62 Warriors likely averaged at least 20 turnovers per game. Between FGA and presumed turnovers, that's 131 possessions before we account for the FTs.
Off-hand, I don't know the exact value of FTs, however I'm fairly certain it would be something like 0.4 possessions per FTA (the reason for that, is to take into account "And 1" plays that would occur in the same possession as a made FG). Under those conditions, 40 FTA would translate to 16 possessions, which brings our grand total to an estimated 147 possessions per game.
125.4 points in estimated 147 possessions would be 85.3 points per 100 possessions (Points/Posessions x 100). That's not the entire league, just the Philadelphia Warriors. This list shows how inefficient the rest of the league was back then.
League Average: .426
For comparison, the league average FG% today is .458 (and that's with players taking low percentage three-pointers). The Warriors and their estimated 0.85 PPP were actually one of the more efficient teams in the league. Wilt only shot 50% from the field, which seems tame by today's standards (and hardly unstoppable), but at a rate 80 points above average, that was as good as it got in 1962.
Effective Field Goal% is a measure that accounts for the value of the 3pt shot. The league average eFG% is .497 in today's NBA. Back in 1962 the 3pt shot did not exist - so the eFG% was .426 (a full 71 points worse than today's NBA).
Taking all of this into account, it is very clear that the NBA stars of the past would have had to work their tails off just to make it into today's NBA. The talent pool of the league was very low, athleticism was very low, heck, even the players were "low". Aside from Wilt, most centers in the 60s were 6'6" to 6'9" and considerably lighter (only 210-235 pounds). Kobe Bryant and Carmelo Anthony would have played center 50 years ago. Minus their incredible athleticism and jumper range, of course.
SIGNIFICANT RULE CHANGES
1980: In perhaps the most important rules change league scoring has ever seen, the 3pt shot was adopted by the NBA.
The results are seen immediately - a jump to 105 PPP and the league never looked back.
1982: Zone defense rules clarified with new rules for Illegal Defensive Alignments. In other words, the death of zone defense.
With zone defenses severely limited, teams began to take the physical limitations of handchecking to the limit. While the Bad Boy Pistons were especially known for this, it did not stop teams from scoring at an efficient rate that was simply unheard of 10-20 years prior. League scoring hit an all-time high 108.3 PPP in 1987, just when the Bad Boys were starting their run.
SIGNIFICANT RULE CHANGES
1994: The 24-second clock is reset only when the basketball hits the rim. Previously, the clock would be reset if the ball hit either the rim or the backboard. This is a huge blow towards offenses, as it is much easier to force a 24 second violation with the change.
1995: There was A LOT going on in the 90s, particularly in 95.
A. The league shortened the three-point line (22 feet in the corners extending to 23 feet, nine inches at the top of the key) to a uniform 22 feet around the basket.
B. Awarded three foul shots for any player fouled while attempting a three-point field goal.
C. Hand-checking eliminated from the end line in the backcourt to the opposite foul line.
D. "Clear path” rule changed to include contact in the backcourt. If a defender, grabs a player when the player has a clear path to the basket on a breakaway, two foul shots will be awarded.
The NBA had seen enough, and eliminated hand checking, while also shortening the 3pt line to a distance so short even players like Alonzo Mourning were shooting threes. Untalented shooters like Tim Legler thrived in this environment, while players like Steve Kerr and Dennis Scott had record three point seasons before the line was reverted back to it's original distance in 1998.
During the 80s, the league was dominated by offensively stacked Showtime Lakers and Celtics. When the Pistons came along in the late 80s, a team had never won by focusing on defense and rebounding the way they went on to do. Pat Riley saw what the Pistons were capable of, and he began to adopt the same physical style of defense when he came to New York. Even though the league changed the rules to eliminate hand checking, the "damage" was done. Teams realized that defense wins championships, and began to build great defensive teams. Scoring began to drop. While the league simultaneously instituted a shorter 3pt line, that only served to temporarily mask the problem. Scoring continued to decline, as defense spread like wildfire. The league hit a 21 year scoring low during the lockout season in 1999.
THE 2000s (first half)
The NBA's decline in scoring continued for 10 years, culminating in 2004. While 1999's scoring numbers could be attributed to out-of shape players and an abrupt training camp, the same cannot be said of the NBA in 2004. There were numerous rule changes in 2001 and 2002, but none of them served to curtail the continued emphasis on team defense. 2004 saw a powerhouse defensive team with no offensive stars DESTROY a team led by Shaquille O'Neal and Kobe Bryant (and flanked by "role-players" Gary Payton and Karl Malone).
SIGNIFICANT RULE CHANGES
2001: Five-second back to the basket rule implemented. This was a slight blow to any team that operated their offense in the post.
2002: The following rules were implemented to limit defenses.
A. No contact with either hands or forearms by defenders except in the frontcourt below the free throw line extended in which case the defender may use his forearm only.
B. Neither the offensive player nor the defender will be allowed to dislodge or displace a player who has legally obtained a position. (They might as well have named this one "The Shaq Rule.")
C. Defender may not use his forearm, shoulder, hip or hand to reroute or hold-up an offensive player going from point A to Point B or one who is attempting to come around a legal screen set by another offensive player.
D. Slowing or impeding the progress of the screener by grabbing, clutching, holding “chucking” or “wrapping up” is prohibited.
2002: More defense rules.
A. Illegal defense guidelines will be eliminated in their entirety. (Hooray for the return of zone defenses!)
B. Defensive Three seconds rule implemented (No more camping in the paint).
C. The time that a team has to advance the ball past midcourt will be reduced from ten seconds to eight (To speed the game tempo up).
D. Brief contact initiated by a defensive player will be allowed if it does not impede the progress of the player with the ball.
THE 2000s (2nd half)
SIGNIFICANT RULE CHANGES
2005: New rules were introduced to curtail hand-checking, clarify blocking fouls and call defensive three seconds to open up the game.
No rule changes have been made since, as this had exactly the desired effect the NBA and it's fans wanted. In one fell swoop, scoring jumped by 4 PPP, and last season returned to the all-time high mark set in 1987.
The removal of illegal defense rules coupled with no hand checking has led teams to play a swarming, rotating based style of team defense where every player on the court is accountable. The Celtics brought this style to the limelight in 2008, and other teams have begun to adopt it as they did Pat Riley's schemes in the early 90s. Time will tell if the offense/defense scales will tip back to the defense.
In the meantime, let's take a moment and appreciate the current state of the NBA. Scoring has returned to it's 1980s levels, and the league has multiple superstars to contend with the 80s triumvirate of Jordan, Magic, and Bird. Defense is a beauty to watch now, as it consists of fluid player movement, rotations, and teamwork rather than just clutching and grabbing the closest man.