I'm stumped. What did I miss here?
In a comment on the original question I said that, among other reasons, questions are incomplete (and therefore fit the definition of “not a real question”) when they show no research effort. Marthaª replied that this criterion “can contribute to ‘not a real question’ or ‘general reference’, but it really depends on the particulars of the case.”
Actually, though, research effort is always required, regardless of the type of question or the particulars of the case, because it is central to the site concept: by and for scholars of the English language.
This is why the first reason given in the tooltip for the downvote button is “this question does not show any research effort”.
The site concept has been articulated and the same point made at various times and places by the site management. Some examples:
Questions at EL&U should be expert-level questions which are “interesting, unique, and thought-provoking”. — Single word requests, crosswords, and the fight against mediocrity by Shog9, StackExchange staff
OP must “have a particularly interesting problem to solve” and put “effort and research into the question”. — Correct word for a design in progress by Robert Cartaino, StackExchange staff
OP can “ask simple questions as long as they are thoughtful, intriguing questions posed as you would ask them of an expert. Overly simple questions without research or forethought should be closed. … There’s no lowering of the bar for a so-called learners’ status.” On the closure of English Language Learners by Robert Cartaino, StackExchange staff
“Requiring the OP to show some research is key, and simple: what have you tried? and in what context do you plan to use this? The sidebar that appears when you ask a question also covers this … [the] relevant bit being share your research. And if you haven’t done any, should you even be asking here?” — Against single word requests by Jeff Atwood, StackExchange co-founder
As I pointed out elsewhere, this concept is also expressly stated in the site charter:
Stack Exchange is a growing network of individual communities, each dedicated to serving experts in a specific field. We build libraries of high-quality questions and answers, focused on each community's area of expertise. (from SE/about)
The English Language and Usage Stack Exchange is for linguists, etymologists, and (serious) English language enthusiasts. (from English.SE/faq)