I think you hit it on the head. As you said, with a Space Opera, you know what you're getting and you can easily suspend belief when needed. I would include Dun

I think you hit it on the head. As you said, with a Space Opera, you know what you're getting and you can easily suspend belief when needed. I would include Dune in this category to a large extend. With Hard SciFi, your "engineering brain" is turned only fully and you enjoy thinking through the actual plausibility of things; I'd include the Martian in that category. I didn't get far in Seveneves, but what I read felt to be uncomfortably in-between. I couldn't tell if I needed to suspend disbelief and look for the less technical meaning, or if I should be having fun thinking about the science/engineering plausibility. Uncanny valley is a good description to me. I think Stephenson found an interesting in between. Not a criticism of the book; just an observation/assessment.