<p><strong>Proposed criterion:</strong></p>

<blockquote>
  <p>The question is about a specific aspect of one programming language or
  statistical software, that does not require statistical expertise to
  be understood and answered. It could be formulated in an agnostic
  manner so that anyone used to that programming language or software
  might provide a definitive answer without resorting to their
  statistical knowledge. <em>It is off-topic on this site.</em></p>
</blockquote>

<p>Examples classified as off-topic:</p>

<ul>
<li><a href="https://stats.stackexchange.com/q/48560/930">Vectorizing Operations on Vectors of Different Length</a> [SO, but in fact there's no statistical context]</li>
<li><a href="https://stats.stackexchange.com/q/48083/930">Boxplots using ggplot2</a> [RTFM, after 2.5 years I'm still looking for a question on how to effectively use the Grammar of Graphics]</li>
<li><a href="https://stats.stackexchange.com/q/47413/930">Is it possible to extract fixed effects in the fixed effects model in STATA?</a> [RTFM or read Stata digest]</li>
<li><a href="https://stats.stackexchange.com/questions/5007/how-can-i-change-the-title-of-a-legend-in-ggplot2">How can I change the title of a legend in ggplot2</a> [RTFM; does not directly serve the purpose of data visualization to answer a specific statistical question]</li>
<li><a href="https://stats.stackexchange.com/questions/47736/how-can-one-convert-multiple-dvs-from-long-to-wide#comment92753_47736">How can one convert multiple DVs from long to wide</a> [RTFM; ???]</li>
</ul>

<p>Examples classified as on-topic:</p>

<ul>
<li><a href="https://stats.stackexchange.com/q/27400/930">Logistic regression: grouped and ungrouped variables (using R)</a> [it requires some understanding of grouped vs ungrouped data wrt. ML estimates or residuals analysis if the OP wants more details]</li>
<li><a href="https://stats.stackexchange.com/q/38616/930">Visualizing mixed model results</a> [you need to be conversant with mixed-effects models to suggest and justify a solution]</li>
<li><a href="https://stats.stackexchange.com/q/35220/930">Removing extraneous points near the centre of a QQ-plot</a> [it is typically part of the questions about statistical computing that we all like]</li>
</ul>

<p>There will always be borderline cases. Seconding @whuber's remark, the community has the means to express themselves to each new question, and many users do that already. I am very grateful to them. If a consensual decision could emerge from this thread, this would certainly facilitate the task of all reviewers on this site. In any case, we need to find a way to move forward.</p>

<hr>

<p>I'm now making a little digression.
It is difficult to delineate a clear frontier between applied statistics and the particular use of a statistical software, much like it is difficult to draw a consensual line between SO and CV when it comes to questions that use R or another software to answer a specific question in a somewhat vague statistical context. Because applied statisticians ought to rely on dedicated software, this site was from the beginning expecting questions related to the use of a statistical software to solve specific tasks. Does this mean that we have to collect questions about performing a left join operation using some lisp flavour with a postgresql backend? I don't think so. 
There are many R-only questions on this site, there are also many statistical questions on SO. An important distinction has to be made, though. This site has evolved since its inception, and so did its general policy on what is on- or off-topic. I believe that it has always been the reflect of what the community is expecting from this site, but <em>I would be very happy to hear from the community</em> in this particular case, and I invite everyone to discussion, here or in a dedicated chat room.</p>

<p>I should note that relying on old threads on our Meta does not do justice to the following:</p>

<ol>
<li>For this site to grow and be accepted in the Stack Echange network, we answered a lot of questions during the beta and just after: I have noticed that, starting in 2011, a bunch of the newly posted questions were about mathematical statistics and applied data analysis, and it certainly helped to attract a new audience;</li>
<li>There has been a noticeable but joint evolution/split of CV and SO regarding R programming: it is clear that many users have turned to SO when it comes to answering R questions, but they are still visiting CV to share their expert knowledge in statistics.</li>
<li>Stack Exchange has evolved itself, with a new reviewing system and the fact that old threads can no longer be migrated to other sites: it is now easier to spot off-topic questions but we can't clean up all threads.</li>
</ol>
