I really like how xenoterracide presented this, particularly putting the focus on who is the user of Unix Stack Exchange. After all, the identity of this site's target users is the question that the first statement of the site faqsite faq is answering.
The second question asked of new site proposals on Area 51 is "who is this site for" but I think it is the most important. The only way these sites work is if you have people willing to participate on them, in particular the people who are willing and able to write answers. From my point of view, a Q&A site like this sets a boundary around its topic because it helps to make the people who answer questions happy.
If a question is something most of the Unix experts frequenting the site wouldn't mind writing an answer to, it's probably on topic. And, from the question side, the site definition needs to make it relatively simple to determine if the question is on topic. I think that xenoterracide statement to "focus on users getting help as best and as quickly as they can" accomplishes that very well.
I use a computer that runs on Linux, I'd like to think that when there is something I don't understand about it this is a place I'm able to bring my questions.
