I have seen a lot of questions lately that will have different answers, depending on whether the user is running Drupal 6 or Drupal 7. Very often the first comment is "What version of Drupal?"
As far as I can see, on meta sites there are tags that are marked as mandatory, such as discussion; if the software they run is the same, then it should be possible to do the same on the main site.
The problem is that the version tags should not be mandatory, but used when strictly necessary. I should not use 6 because I am using Drupal 6, but because the question (and the answers) is only valid for Drupal 6. When somebody see the question, and see it's tagged 6, he could decide not to read the answers because he is interested in Drupal 7, but the answers could still be valid for Drupal 7. I could ask a question that doesn't depend from the Drupal version; if you force me to select a version tag, then I should use at least two. This would mean that, when Drupal 8 is out, somebody would read my question and, seeing it is tagged 6 7 could think it's a question for those Drupal versions, when it is not so.
It is not different from tagging a question c#-4.0 when I am asking the syntax to use for the "switch" statement, which was not introduced in C# 4.0. You could object that in Stack Overflow you could tag the question with "c#," but the equivalent tag in Drupal Answers would be "drupal" that you can consider an invisible tag always added to every question.
Some questions can be version independent. Well, most questions that does not depend on actual api should be version independent. And if question have different answers for different versions, nothing prevents posting them all.
Mechanism detecting phrases like "Drupal 7" in body or title and suggesting appropriate tags would be nice, but I think we may do it by hand easily enough.