For me, what I've noticed is that SO doesn't really allow opinionated based questions, meaning there is no room for any gray area, while here on HN, I think not only is it allowed, but encouraged.
For example, here is a question that would almost certainly get down-voted within 5 minutes of being posted:
Should I use jQuery or vanilla javascript in my small side project?
There is no right answer here, it's all how you feel. Those kind of questions are frowned upon on SO, while on here you could have a healthy discussion going on days on the pros and cons of both.