By "primary tag" for a question I mean:
- The tag used in the
- The tag used when automatically tweeting questions
- The first tag in the tag list for a question
Currently, the primary tag is the tag with the highest question count as a byproduct of the fact tags are sorted by question count. While this works most of the time, it often doesn't and leads to all kinds of issues on some sites (cough).
My proposal is based on the assertion that the tag you write in first when asking a question is what occurrs naturally to you, the asker, as the one tag that best describes a question. It would then be natural to acknowledge this and use that one tag as primary, and sort the rest by popularity.
For example, if you're asking about how to use Python libraries from inside Haskell (silly example, I know), your primary tag would be Python by virtue of view count. Logically, however, the question is about Haskell extensibility, not Python. You could solve this by not tagging Python, but that'd be silly: the question is also about Python.
Doing this also opens the door for some interesting analysis for say, better detection of broken windows in a site. If anything was to happen to the tag you marked as primary, your question would probably need to be looked at!
If that tag vanished as a matter of policy change, it would probably mean your question has just became a broken window that needs closing and/or deleting with the optional polite message. If it happened because the topic you're asking about is so obscure you only got one question for it in the last six months (it happens!), your question risks losing an essential tag without becoming untagged: an hidden broken window.
So that's why I believe the primary tag for a question should not be the most popular tag, but the first (non-required) tag you write in first in the "tags" section.