This encompasses all previous questions. I decided I didn't want to say this more than once, and I've changed my stance a bit.
For the most part it's unix vs windows.
IIRC Windows uses threads where unix forks. This means that Settings for servers like Apache may vary in some settings, and even in the way it should be compiled. This also means that Languages like Perl have to work differently on Windows.
Bash and Zsh which would probably be considered undeniably unix can be run on Windows (even if it means the presence of a cygwin.dll )
Firefox has its preferences menu in a different place on Unix, so who's to say that Something as generic as a browser isn't a valid *nix question. Firefox was *nix first really.
I can install a great number of apps that are primarily *nix on windows now, does this mean we shouldn't allow questions about them here?
I think the separatism needs to die, this site isn't about what should be on SO,SF,SU vs what should be on Unix SE. This site, like those, is about helping each other with our problems.
Sure this may not be the best place to ask about Apache, but I think I'd rather tell the person "you'd get faster answers on SF" than I would saying it's off-topic. Apache works fine on Unix, just don't ask about IIS.
Technically it might even be ok to ask about windows apps, we have Wine (and derivatives) right? So long as your question is about running them in wine (or some other way related to *nix).
We don't have to limit Unix to SUS (Single Unix Standard) I've asked plenty of KDE questions on here, and for those who don't know some of KDE works on windows.
Ultimately the point I'm trying to make is that the lines are blurry, let's focus on users getting help as best and quickly as they can, which may mean recommending another site (or migrating questions once we can ), instead of arguing about whether something is or isn't Unix enough. When in doubt, it's Unix.
stepping off soap box
as a non argumentative response to Michael, this following diagrams are a visual representation of what shapes my opinion. Although I at first generally agreed with his opinion I realized that there's a certain problem with it on scale.
The following Image is a figure representing Non-Unix and Unix circa 2000 Maybe, as you can see, lot's of Non-Unix (windows) and not a whole lot of overlap. This is not based on any real statistics and was POOMA.
This next picture is representative of where we are today ( Including OS X and Android )
You'll notice that what is pure unix hasn't grown much, but now there's a lot more overlap and a lot of growth. This comes from many apps not being cross-platform. Back then QT was Unix only (I think) now it works on every platform under the sun, Perl's windows implementation was quite different from the unix version, now we have Strawberry. Zsh runs on windows, MSysGit is coming along nicely, VLC and MPlayer (+ gstreamer) are everywhere, Firefox is the #2 browser and runs on all platforms, I've even used Konversation as an IRC client on windows.
I would like to see our circle continue to grow, and in many ways I think the less differences is a good thing.
I also think that the fact that we've been having this opinion that we won't help with resources that also work on windows could be some of why Ubuntu SE wanted its own community, although I haven't done any research to back this up, I'd be they have no problems with helping a Ubuntu user back up his firefox bookmarks, though they might say take the details of using XMarks to (webapps).
In order for our circle to grow we must be willing to come out of our Ivory Towers to help the unwashed masses on the path to enlightenment. This is not to say we will not migrate questions elsewhere once that becomes possible, it's about helping users after all. If their question is better answered on SO (etc) then we should migrate it there, but I'm not going to try to figure out if questions are 'unix enough' anymore, best simply to suggest that something is better asked elsewhere ( like this ruby question ) than to get in a debate about whether that tool is unix or not. I'm more interested in seeing that user get help.
As far as Ubuntu though... Many of those users wanted a separate community, I would see any effort to migrate questions to ubuntu SE or from ubuntu SE as an abuse of users that chose specifically to post in one community or the other.