No quick answers here. There is no clear template to apply (thank heavens!), which dictates which elements of a story must be tidied up, and which are allowed to flow on, drift toward a yet unwritten future.
But how can you decide?
It helps to understand how words work.
Words move us. The communication of ideas happens most effectively when multiple senses are employed in the reading of words. We read a phrase:
She washed berries by the handful.
And it sends a picture - no, not a picture, but a multitude of pictures nearly simultaneously - until a favorite picture takes precedence.
We picture "she"
Not any "she". Our "she". The best "she" for us. The one we want to picture.
We sift through our ideas, our notions of "she".
We settle on the one we want her to be.
We picture "berries"
Our favorite - red and perfect.
Or exotic and wild.
I see in my mind the ideal berry. I see what "berry" means to me. I imagine what it means to you.
. . .and "handful"
Two full hands, or one? Rough or smooth. Stained or wet.
. . .and "washed".
. . . from a garden hose or a kitchen sink.
We settle on one aspect of the image, then another, rapidly viewing, discarding and deciding on components of the scene until our mind forms a complete whole. The process takes milli-seconds.
This is why the question of loose threads is difficult. Because irresistible fiction becomes a nearly endless procession of deeply personal images in the minds of each reader.
We take the words on the page and recreate their meaning in our minds - and the images we form become the truth about the book. They become the book's meaning.
The author then, must think about the best possibilities the best collection of ideas to wrap up, complete and finish off in the book - and then must consider which threads to leave loose, which elements of the story to leave untamed in order to best serve the story and the reader.
It's not a crap shoot - it's not a guess. It is instead the writer's artful dancing with the reader she loves - they dance to the music of the story.
We do the same thing with a story - when we read, we begin to "own" the story. We recreate it in our minds. And we build expectations in our mind about what we need to know (to be told) and what we already know (want left latent in the story). What we want to have happen, and what we are willing to let go of.
Loose thread should never be an excuse for not finishing a story - rather they will, by the images they create, the feelings they evoke, reveal themselves to us - but only if we dance to the music long enough to learn all the steps.