Story > The story is very different. Time of troubles, Bhaalsapawn, mindflayers are a central part of BG1, but are optional enemies on most of the BG2 part.
Items > BG2 has more depth on items. Clay golems require blunt magical weapons, plate armor have higher AC vs slashes thank blunt and makes sense that maces > swords vs armor
Magic > There are over 300 spells on BG2 and each one is unique. The magic system is far more old school. Spells are far more deadlier, but remember. Every spell which an PC can cast, an NPC can cast.
