- "Too open-ended": long answers are welcome on the site, but a question should not require an overly-long answer. A typical "answerable" question on this site can be answered, by an answer which consists of just a few paragraphs with maybe a reference to a canonical literature or a book. The rare question which would requires a 1000-word essay as an answer is therefore off-topic. You can ask broad questions (if a shallow answer is OK), or a detailed question if it's focused on one topic and answerable.
Similarly a question which most likely requires a back-and-forth discussion, perhaps because almost any answer will be seen as unsatisfactory, would also be too long for this site.
For example, "please explore X and tell me everything you can about it", makes for a good homework assignment but not a good (finite) topic for the "Question & Answer" format of Stack Exchange.