Since 1990, the NBA has instituted a series of rules changes to increase the offensive player's flow and make physical play costly. First came increased penalties for flagrant fouls (1990) and fighting (1993), the implementation of the "five points" rule that called for automatic suspensions of players who amassed a certain number of flagrants (1993).
Hand checking was eliminated in 1994. Using the forearm to defend players facing the basket went away in 1997.
In 1999, the league eliminated contact by a defender with his hands and forearms both in the backcourt and frontcourt, except on offensive players who caught the ball below the free throw line extended. Defenses were also prohibited from "re-routing" players off the ball. This freed up perimeter players who used screens to get open. Nor were defenders able any more to grab or impede offensive players setting screens. In 2001, the defensive three-second rule eliminated defenders camping out in the lane away from their offensive man to help.
The rules changes did what they were supposed to do -- open up the game. Scoring average has increased from an average 95.6 points per game in the 1997-98 season to this year's 100 per game. Overall field goal percentage has increased from 45.0 percent in '97-'98 to 45.9 percent this season. Three-point percentage has gone up, from .346 11 years ago to .367 this season. And fouls have gone down, from a league average of 1,837 fouls in 1997 to 1,726 this season. The statistical-based Basketball Prospectus wrote at the beginning of this season that the game's pace -- defined as possessions per game -- had increased from its nadir during the lockout season of 1999 (around 88 possessions per game) to around 91 per game in the 2007-08 season.
http://www.nba.com/2009/news/features/david_aldridge/04/22/aldridge.defenses/