See deleted answer: http://stackoverflow.com/a/16495860/41403
I appreciate the moderators have a thankless, unpaid voluntary task. But over-moderation is a great way to turn this site into another wasteland: contributors feel it's not worth the effort, and stop giving.
Since no reason is given by moderators for a delete, we have to play guess-the-rule with the FAQ:
•commentary on the question or other answers
•asking another, different question
•“thanks!” or “me too!” responses
•exact duplicates of other answers
•barely more than a link to an external site
•not even a partial answer to the actual question
The first four don't apply, so we assume it must be "barely more than a link to an external site".
Okay, this is possibly true, depending on the definition of "barely".
The external link is an extensive article to an external, trusted source. It covers the issue the question posed and adds value to the other answer by providing a wider context for the questioner. It is useful, it is not off-topic, it is concise, it is well-intentioned.
I'd like to hear from the mods what they think my answer should have been? A long-winded précis of the article? An explanation of the content? Should I just plagiarise it?
So perhaps it's the rule that's wrong here: perhaps "barely more than a link to an external site" is a valid, useful answer?.