Post History

The main guiding principle should be what fraction of posts a typical user would find interesting, or at least feel are related enough to their interests to not be annoyed at them for being off-topic babble.

Individual users will have a more narrow focus of interest than a whole site.  They will get annoyed and leave when too much of the content isn't interesting to them, or at least doesn't feel like it fits.

And no, tags aren't really solutions to this.  At least in my own experience, I never used tags to filter content.  Doing so feels like you could miss out when something interesting gets miss-tagged.  It also feels like hiding the problem instead of fixing it.  Suppose you could put on magic glasses so that you don't see dirt in your house.  You may not see it, but it's effects are still there, and you still need to clean it.

A good example was the SE Outdoors site.  That's a wide topic, with many sub-topics that don't interest me.  As long as the posts I wasn't interested in weren't overwhelming, I didn't mind.  I even liked to see what some of the other topics were about.  Then a new user came along and posted several questions a day about archery.  I'm not interested in archery, but don't mind reading an occasional post.  However, this flood of archery questions made everything else move off the front page quickly, and made the place feel like it was only about archery.  As a result, I gave up on the site for a while.  A year or so later when I took a look again, the topics were more mixed, so I started visiting more regularly.

So, keep sites reasonably narrow.  The two factors that push against that are:<ol>

<li>Not enough users would be active for a niche sub-topic.

<li>Multiple sub-topics are related in that answers for a question in one often would get into the others anyway.

</ol>

Of course these things will always be subjective, so there's no way to make a solid rule about how wide the topic for a particular site should be.