Thanks for the informative reply!

I think that, for someone who primarily uses Ni-Fe, the understanding or awareness of someone's motivations would be more automatic, and largely unconscious.

I think the way I worded it may have been misleading. I don't think I'm actively trying to figure out motivations, at least not 100% of the time. But I think the intuition also scales with how well I know the person. For example, I can immediately recognize how and why my brother behaves the way he does because I've lived with him for almost 20 years. Give me a stranger and I can probably do the same, but it's not as immediate. Would you say that as an INFJ, these impressions come to you almost immediately?

When you say:

It's not hard for me to "read between the lines."

That's also something that I find myself agreeing with, but this too is highly dependent on the subject. If someone is talking to me about politics it's usually quite simple to understand the underlying agenda, but a conversation topic that's less emotionally charged will be more difficult to read. How does an INFJ deal with idle banter in general? My impressions from the threads I've read here seem to be quite similar to the things being talked about in /r/istp as well as my own experiences. Though I can't tell if this is more of a general anxiety thing.

To your last point:

A puzzle is a good example of how Ni-Se works, really: the more pieces (information) I have, the clearer my understanding of the image (idea).

Essentially, you are okay with having a half-formed idea/project out there so that you can build upon it later, whereas an ISTP prefers seeing only the finished product? Too much of a generalization? But then you mention:

All of this isn't to say that I don't need to work things out in my head, because I do - that just isn't the first thing that I do. Impressions first, organization later.

Which is something I relate to as well, though again it can be quite situational. I'm a bit confused about what 'impressions' refer to. Is it raw sensory information, or are they your filtered 'opinions' on that information?

All of this does seem to point towards ISTPism - the stereotypes are still really throwing me for a loop because I would have expected to be able to relate to at least some of them. In the end stereotypes are made to be broken, I guess.