re: How is today's NBA style of play different than 20, 25 years ago?

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There are two illegal defense rules. The old one prohibited zone defense. The new one limits the ability of a zone defender to remain in the paint guarding no one.
For decades the NBA required teams to play man-to-man defense, which meant everyone on the court had to guard someone. You could double team one player and then recover to your man, but you could not be caught between players, guarding no one. That would be an illegal defense. That also meant you couldn't just stand in the paint all day guarding no one.
To take advantage of this rule, lousy teams with one great player like Michael Jordan would put MJ on one side of the floor and everyone else on the other side. This was the isolation play, or iso, for short. It would turn into a game of one-on-one which MJ would win. (This is before Jordan found some worthy teammates and Phil Jackson incorporated them into the offense with the triangle.)
Various tweaks to the rules tried to limit isos, but never eliminated them. When zone defenses became legal, though, it made isos much harder. The two man game, the pick and roll or pick and pop, has become much more popular than isos, simply because the defense can easily flood the side of the floor where the iso is taking place, forcing the star player to go one on three or pass the ball.
The NBA tried to give something to iso players when it eliminate the old illegal defense rule and made the zone legal. In order to prevent rim protectors from standing in the paint all day, when the NBA eliminated one kind of illegal defense they created another, the 3 second rule. A defender cannot stand in the paint guarding no one for 3 seconds or longer. Every 2.9 seconds he has to step out of the paint. That's the modern illegal defense rule.
However, despite the new illegal defense rule, legalizing the zone defense led to Tim Thibodeau's ICE strong-side overload defense, popularized when Thibodeau was the assistant coach in charge of defense during the Celtics' championship season in 2007-08. The zone had been legal for a while, but Thibodeau figured out how to flood the strong side of the floor, the side with the ball, with an extra defender, often forcing a long cross-court pass that would allow time for the defense to recover. The object of the defense was to force the ball out of the best player's hands.
Thibodeau dealt with the new illegal defense rule by training Kevin Garnett (and later Joakim Noah) to remain in the paint for 2.9 seconds, step out, then step back in. As long as he steps out every 2.9 seconds, he could be in the paint all day, guarding no one.
Thibodeau's ICE strong-side overload defense, or variations thereof, have been widely adopted in the NBA. The iso has fallen out of favor. The two man game is very popular, but passing to the other side of the floor must remain an option if the two men on the strong side find themselves outnumbered. One of LeBron James' remarkable abilities, for example, is making bullet passes all the way across the floor with precision to a spot up shooter standing at the three point line, so that the defense doesn't have time to recover. This is a vital skill under present rules.
Edit: /u/chingt raised an excellent point I forgot to mention and want to highlight:
Under the old illegal defense rules, you were not allowed to double a player without the ball, so say you're guarding Luc Longley, and he's standing on the weakside at the 3 point line. You see Jordan setting up to catch for an ISO, and you know Longley isn't going to shoot a three. Unfortunately, you are not allowed to leave Luc until Jordan catches the ball or you will be whistled for illegal defense, giving the Bulls a technical free throw.
This basically guaranteed that on any isolation, you could have a second or so to attack against single coverage. For great iso scorers, this was often more than enough to get yourself in a position where no amount of help defense could stop you.

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