Well, I did hear that Bioware learned from what Black Isle did with Planescape and improved on BG1 in a number of areas where they were weak.
One of the main things I hated was that your party was made up of empty character husks who were no more than their four or five lines that they spewed out constantly. Minsc was amusing for the first hour, and then he was the same for the rest of the game. Same with all the others, really.
In Planescape I could actually talk to my party members and I found a lot about them in long, deep conversations. I could have detailed conversations with NPCs in the world to find out more about the world and about various factions and characters. You couldn’t really do that in BG1, but I heard they changed all that in BG2. That you could talk to your party members at regular intervals and have long detailed conversations with them. That you could have much deeper conversations with NPCs out in the world, and find out more about the Sword Coast than the cursory stuff from BG1.
And since you weren’t low level characters anymore, I heard there was a lot less saving and loading required, which is the activity I did the most in BG1. If I were to describe BG1’s core gameplay, it would be hearing my party members say the same lines over and over and saving and loading and watching a rotating hand turn to dust. That was 90% of BG1.