Prior to Baldur's Gate, many games were made based on D&D, and most of them were VERY faithful adaptations of the 1st and 2nd edition rules. Including the 10+ Gold Box games and the two Dark Sun games, all of which hewed quite closely to the parent ruleset and gameplay style.
Then BioWare got their chance to make a D&D game. And they decided, based on their own personal opinions as video game designers, to make significant changes to the system because they felt that it would be more fun (or sell better, some of both probably) than keeping their game as tight to the 2e rules as the prior D&D titles. And now Larian is doing the exact same thing.
But on Reddit and on the Larian forums, we've got a lot of people pretending that Baldur's Gate 1+2 was some kind of deeply faithful adaptation, and Baldur's Gate 3 is a wild departure. I think people are kidding themselves.
Baldur's Gate took a game in which each character takes their turn individually, in order, and changed it into a wild melee where everyone acted at the exact same time. That is a HUGE change. The impact on how fights play out in such a system is quite significant, and they way it FEELS to play it is even more massive. The ability to pause and issue commands does not change the fact that any time you have it unpaused, everyone is acting simultaneously. That's the single biggest change to D&D in a video game of all time.
In addition to this, Baldur's Gate 1+2 changed the way a number of spells worked, to fit better with their new system. I can't tell you exactly how many, as I'm not prepared to comb through all the spell descriptions for an hour to gather that data. But it was a not insignificant number.
Eh, that's overstating the changes Bioware made to 2ed rules. There is a misapprehension that 1 and 2 were 'Real Time'. But the actual rules changes Bioware made were quite minor. 1) They replaced party initiative with character initiative. 2) They reduced the 60 second turn time to 6 seconds. So characters in those games are actually staggered on a delay ranging from 1/10 of a round to 1 full six-second round. They tuned movement rates so all characters could cross the width of the screen (at old school resolutions and aspects ratios) in 6 seconds.
It makes the game a lot more fluid, I agree, but it's not at all a huge departure from the core 2ed rules.
