This is a first, but I thought I'd answer the question in a post, since I'd like to get some other opinions on this. I don't want to give the wrong answer. First, here's the email I just found in my inbox:
Hey I just found your site today and I love it. There's so much to read. From your posts is seems you've got the Korean language down pretty well. I'm a Gyopo and can speak at a good intermediate level and I'm wanting to continue learning. But I've been looking for someone to ask some basic questions. There are some basic grammar questions I want to ask someone who's basically fluent in both languages (which is actually hard to find)....because I know when I speak sometimes I don't say it the way a korean would say it. Just for example.....in your mind how do you distinguish and decide to use -니까 and -어서. I've read several books and they seem to contradict each other and not make sense. Just from my 'feelings' I think of the former as "because" and the latter as "so" but it's hard to say when youc an use one and not the other. Anyways, if you're not the right person to ask or if you don't have time I completely understand. For some reason I felt I should try asking you after reading some of your articles.
Well, in my head, I kind of have the feeling that when I use -니까, it's a strict "because" tied with the fact that the person I am speaking to already knows the fact in question, or that they reasonably might. When I use the 어서 form, I have the feeling that it's more of a flowing "cause and effect" as part of an explanation. And in the big picture, I use them somewhat interchangeably.
I can't go because I'm too busy. / 너무 바빠서 못 가.
(As you know) I can't go I'm too busy. / 너무 바쁘니까 못 가잖아. (Or it's a more "direct" way of saying the fact...)
(I said) I'm too busy! / 너무 바쁘다니까!
Because I'm too busy. / 너무 바빠서.
I displayed this post on the projector in my class, and my students all agreed with the interpretations here. I hope this is helpful!