StackOverflow isn't Jeopardy. Questions don't necessarily need to contain question marks to be understood and answered. I also think that questions shouldn't need to contain question marks to be postable.
If we turn them away completely by saying: "You didn't include a question mark so you can't post here", we're essentially saying "We don't want your kind here".
If instead, we allow new users who do not understand the system to post their "questions" and have community members point these users in the right direction, the system is more inviting, and gives the impression of "Thanks for giving it a go, we're all here to help you".
Certainly some users will never learn from their mistakes, and keep posting poor quality questions, but I would hazard a guess that they're in the minority. Questions that are good enough will be upvoted, and if there are improvements to be made that are worth making, members of the community are generally pretty good at stepping in and lending a hand. It's why SO is so useful. People actually help.
Addressing Update 2:
If you disagree that absence of a question mark is a strong signal that the question is poor, could you please provide examples of good, useful and appropriate questions that do not contain question marks?
I hold my judgement on whether there is any correlation between the absence of a question mark and the quality of the post. The onus is on you to prove that there is a correlation via evidence. In the absence of any evidence, I cannot say one way or another.
I've provided several additional examples of when the signal works in the comments, and more come up regularly, so I would appreciate to see some evidence of the contrary.
You have provided anecdotal evidence, which neither strengthens nor weakens your argument. Asking for more anecdotal evidence is not an appropriate way to proceed in bolstering any argument.
If it's simply evidence that you want, it would be appropriate to query the SO archives for the data:
All questions should be broken down by two categories into four groups based on their first draft (remember, this whole issue is to address the initial posting of a question, not the subsequent revising).
The categories are:
- Questions with first drafts that contained question mark characters vs those that did not
- Open questions vs Closed questions
The groups are:
- Questions with first drafts that contained question mark characters (
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) and are still open
- Questions with first drafts that did not contain question mark characters (
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) and are still open
- Questions with first drafts that contained question mark characters (
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) and are now closed
- Questions with first drafts that did not contain question mark characters (
?
) and are now closed
If you can provide evidence* that the lack of a question mark character in a first draft strongly correlates to the question being closed, then I will be more than happy to delete (or modify) this answer and create a new answer that supports your proposed change.
* Please also be sure to provide the steps you used when gathering the evidence so that we may reproduce it
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as an operator.?
operator if that's your only objection :-)