In a game like Mass Effect, the variables carried over from 1->2->3 are legion, so to speak.

In BG, they're pretty much non-existing. There are scant few variables, mostly to do with which items are on the character transferred.

If you carry Drizzt's weapons etc. in your inventory upon transfer, he'll assume you attacked him in your BG1 encounter, when you meet him again in BG2, and it shows in his dialogue.

Also, there is a handful of magic items from BG1, and if they are in your inventory at import, they will show up at certain locations in SoA in a prioritized order.

That's pretty much the span of imported differences. To my knowledge, very few, if even any, other variables come into play beyond that one specific encounter. None that I know of, anyway.

However, those are minor points IMO. The real difference between an imported or new character are their attribute scores.

An imported character can potentially start off with larger than life stats. Sure, a new BG2 character can eventually obtain similar "metahuman" stats come ToB, but overall you'll want them from the get-go.