1) engine speed. BG2 is just faster, which means it takes less time for you to explore an area, get from point A to point B, etc. (there may be a way to change this in the ini file for BG1, but I can't remember)
2) Containers - Scroll Cases, Gem Bags, Etc., there are just too many of those items you come across if you don't have the containers to store them
3) the Tab key. Given the vast number of barrels, chests, etc. that are just background and not actual containers, it's a real pain to hover over each one to find stuff. Love hitting tab and seeing everything I need to check into.
I can't speak for item 1, but BG1 Tweaks has a Portable Containers component. They're bottomless, but they do have the problem of removing items one at a time. This can be a real pain for arrows, bolts and bullets. As for item 3, Tales of the Sword Coast enabled that very hot-key.