BioWare built Baldur's Gate II by compiling every criticism of the first game and systematically answering it — and because the Infinity Engine already worked, the entire team could spend its time on content instead of plumbing.
Follows: Baldur's Gate
Most sequels are built on intuition about what worked. Shadows of Amn was built on a document. BioWare gathered the complaints — from players, from reviewers, from its own staff — organised them, and derived a feature list intended to address them one by one. Higher resolution, better pathfinding, more interesting classes, deeper companions: each was a response to something specific that someone had said about Baldur's Gate.
This is an unglamorous way to design a game and an extraordinarily effective one. It works only when the foundation is sound enough that the criticism is about refinement rather than fundamentals, and Baldur's Gate — for all its rough edges — was structurally sound. The sequel could afford to be an exercise in polish because the original had already proved the concept. That the outcome is widely considered one of the greatest RPGs ever made suggests that listening carefully and fixing things methodically is underrated as a creative strategy.
The single largest advantage Shadows of Amn held over its predecessor was invisible to players: the Infinity Engine was done. On the first game, designers built content against a moving target, and programmers built functionality without knowing exactly what it would need to support. Every hour spent making the engine work was an hour not spent on quests, characters, or writing. Starting from a complete engine inverted that equation entirely, and the sequel's vastly greater volume and quality of content is the direct dividend.
Nowhere is the dividend clearer than in the companions. Giving Minsc, Jaheira, Viconia, and the rest real personalities — banter with each other, objections to the player's decisions, relationships that deepened or soured over dozens of hours — is expensive in exactly the resource a working engine frees up: writing and scripting time. The party members of Baldur's Gate II became characters people still discuss decades later, and they exist because nobody on the team had to spend that year making the game run.