Q:

Tags for variant-agnostic questions

There are currently a number of questions tagged unix.  I think this tag is meaningless since every question could have it. ( Then again, so could the computer tag on SU!) 
We could ban the unix tag (I think the SE software permits this), or give it a useful meaning (such as questions requesting portability accross variants).  I lean towards the former because of the potential for confusion.  Then what is a good tag for questions expecting a portable answer?  portable (ambiguous)?  variant-agnostic (hard to guess)? 

I've retagged a few unix questions — mostly the ones where there was or should have been a variant tag as well (solaris, linux).  I think some of the remaining unix tags are superfluous (the ones about unix history make sense) but won't go further there's a consensus (which there clearly isn't now). 
I've created distribution-choice in the process (there are probably more questions where it would apply) — it seemed appropriate, but feel free to argue.  I've also created architecture; maybe design would be better, or something else?  Ditto for bibliography. 
I couldn't come up with good tags for a few questions:

Quantify unix responsiveness
User's Login date and login time (something about user accounting?) 
X-based email for reading mail from cron jobs (mail-user-agent = mua, but askers might not know the term)

Update: The unix tag is now banned. 

A:

On this question we discussed the [linux] and [unix] tags being used to mark things that are specific to those OSes; [linux] posts only apply to linux, and [unix] posts only apply to unix.  In practice that hasn't been happening; we even had a bunch of posts tagged with both, which makes little sense, but xeno went through and retagged them all
It is possible to blacklist tags as a last resort, but I'm still hoping enough of the community will handle retagging that it won't be an issue.  It might be worth adding something in the tag wikis for those tags, but I don't think it would help much
I don't think we should overload those tags to mean portability though; if a question is distro-specific I would expect it to be tagged with that distro's tag and with [linux].  Ideally I think tagging with the distro tag would automatically tag with [linux], since I can't see a way the former would be useful but not the latter, but there's currently no way to have automated tag dependencies (although it's been discussed in the past).  I think your first example, [portable], might be a decent tag to represent portability

A:

I've created a portability tag, for those questions where portability is a key concern.  For example, grep recursive → Searching for string in files recommending grep -R, while grep recursive portability → Search string in many files on HP-UX recommending find -exec grep or find | xargs grep.