I just put here some food for thought, if someone is able to get something from it, great! Thanks for the attention, rich comments and votes.

More incentives to closing questions sounds like a bad idea in the long term to me. What makes the content growing in this site is primarily asking, and current (not future) users are becoming disencouraged to do that - no questions = no answers. Or even worse, only newcomers are encouraged, since they still think it is worth to ask their first question. Unfortunately, newcomers often ask bad questions. So, the whole system can be working like a filter towards a general decrease in the quality of questions, assuming there will be a permanent influx of new users.

Actually, the most obvious look-at-the-manual questions seem to be the ones that best fit the informal (or even formal) requirements of the culture (or rules) here. Luckily the answerer tries to teach something, but could be just shortcuts to information:

question -> manual

Once someone gets their own question closed, this act might be perceived as an 'aggression' - specially if the question is valuable / well worded / according to the rules in some sense. Being closed is not an offense to many people who realized they should have low expectations here. This is valid even for that bad OP that has almost nothing to loose just throwing away questions with no research effort. But there are elaborated questions that have some weight against (sometimes personal interpretation of) rules.

As an aside note, I personally think the best content is the rich one, not the obvious one. The great questions here are from the time SO allowed open ended questions. I may be missing something, but, in the extreme case, if two or more people consent to really learn something in depth... how does this affect the performance of the site? The fact they are called "not constructive" is ironic. Quora doesn't have even space for details (only comments). Reddit, ok, there is Reddit. SO could have an agreement with them to redirect good unfit questions automatically. [end of aside note]

The point here is that there are not only questions perfectly fit into the norm (say 10%) and bad questions (say 70%) - being a generous shameless guesser. There is much room in the middle, and askers are human beings. Take this into account for the content that follows.

From Freudian psychology, the concept of displacement explains how a perceived act of aggression leads to the replication of such behavior onto other OPs. I am just speculating a possible component in a complex web of social relations, but this could partly explain the current overall toxicity some people feel increasing in this site. None of the 10-20 people I worked personally with has the opinion that SO is a friendly place (to put in mild terms).

Other communities are not perceived this way. Anyone who uses other places like Tex, Math, Physics, anything besides SO (and maybe programming) realizes that.

I felt by myself this phenomenon playing Counter Strike some time ago. I always ignored pools about kicking players. Until the day I was kicked for no reason. It was really unpleasant having my play interrupted arbitrarily. But ok, I thought that ended there. The fact is that in the second or third pool that asked me to kick somebody I felt compeled to do that. Even without checking who was the target. That was sick, animal, that was human.

This is really a subject of research, perhaps with some components similar to the Stanford experiment, when sadism was a driving force among the moderators of the fake prison.

Of course there are thousands of useless questions, but closing a possibly good question is like condemning an inocent (or at least all the work put into asking). The threshold should avoid false positives ("close a valid question") much more than false negatives ("keep open a bad question"). It is easy to spot an absolutely useless/spam/lazy question. But if you have a single hesitation about it, then it is enough to keep the question open.

Now, a little bit of rant, just in case someone is interested in getting impressions left by the site:

People sometimes seems crazy about closing questions here. This seems specific to SO, a cultural thing. Rules are there to guide us, not to oblige. You are not obliged to hit your car in a truck because the semaphore is green. You have a good enough intuition to wait for the truck driver to fix the wheel and liberate the path.

Just look at the negative votes in other pro-OPs answers here and the ones I will also get. Unfortunately, I don't have high hopes this community can improve. Like any sect, rules above intuition and reason put people away.

Given such specific behavior found in SO, and not other communities, an interesting hypothesis is that programming too much train our brains to the point we learn to have some compiler behaviors, looking for patterns, taboo words and acting motivates for whatever reason while shielded by rules. It feels good to be able to act in the world, but it is also tempting when it is at our reach the possibility of closing questions. Maybe it is an easier reward than just leting it go, or finding something to answer.

Anyway, thank you who helps removing bad content from the site.

I think different levels of "offender" questions should be considered. Spam being the worst one, then lazy askers, ... etc ..., the minor offense would be to ask an original question without an obvious answer in a software manpage.

ps. Please notice the lots of attenuating, non-generalizing words I've put in here; I intend mostly to stimulate some ideas, despite being human and susceptible to rant.

I don't know how rich (arguably) "unfit" questions can interfere on the functioning of the community, so I really hope people are mostly thinking about spam when they say, like the most voted answer: