Perils of the Writer: Over- and Underexposition

We're all familiar, especially in genre fiction, with the Infodump.  The paragraphs on end describing some piece of backstory or worldbuilding information that may (or may not) be relevant to the reader understanding the story.  This is the sort of thing that can easily go too far, as highlighted by the satirical story "How David Weber Orders A Pizza":

120 meters later, he pressed the central pedal on his floorboard and the vehicle slowed smootly to a zero-zero intercept with the pavement. A signpost stuck out of the concrete outside, in front of and slightly to the right of his vehicle. It bore a large red octagon, with the word "STOP" emblazoned upon its center in white sans-serif lettering. The sign was a way of establishing right-of-way rules at this intersection of two streets. The vehicles going his direction, and going the opposite direction on the same street, would both see these "stop signs", and would thus be required by law to stop before proceeding. Vehicles following the street that crossed this one, on the other hand, would not see any "stop signs." They would be allowed to continue across the intersection without changing their velocity. If neither street had displayed any "stop signs", then vehicles following either street could legally cross the intersection at constant speed, resulting in disaster should two vehicles from each street be converging on the intersection at the same time. With the stop signs, the vehicles following the stop-signed street would be required to stop and wait for any such "cross traffic", thus ensuring that both they and the crossing vehicles would emerge from the intersection safely.

As absurd as that is, a lot of genre fiction gives the same level of descriptive attention to the things that would be mundane in their world.  However, judicious editing can easily help solve that problem.  The inverse problem is a little more challenging.
I've talked about the "iceberg rule" before-- keep 90% of your worldbuilding under the surface-- but at the same time you don't want your work to be impenetrable because you've kept everything all to yourself.
I watch several independent movies-- most of which tend to be character studies rather than structured stories*-- and one thing I've noticed as a recurring problem is a lack of proper explanation of character's situation or motivation.  Of course, you don't necessarily want everything spelled out for you, but you see that overcorrection plenty of times, so nothing is explained and the why of one event to the next is utterly opaque.
I understand why this happens, of course, be it in film or prose.  Writers live with their characters and worlds so deeply that why they do what they do and how things work are so ingrained that it's as clear to them as why you stop at a stop sign.  It becomes challenging to step outside themselves and see how it looks to the outside audience.  More than once I've had my beta readers or editor say, "You can tell us more about that."
Which direction do you err in?  Too much exposition, or too little?

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*- Which is its own topic.