The site tour's assertion that "we're working together to build a library of detailed answers to every question about English language and usage" is clearly not the aspiration of the actual site, as we users administer it.
The split between theory and practice is largely a consequence of the unreasonableness of the goal of "answering every question about English language and usage." For example, any question about what a word means is a question about English language and usage—and yet we reject any question that a person could answer by looking up a word in a standard English dictionary. Likewise, many English class homework questions are questions about English language and usage, as are millions of questions about, say, subject-verb agreement, each one unique in its particular details ("asparagus is" or "asparagus are"?).
We fairly consistently (and rightly, in my opinion) reject such questions, although we don't always clearly explain why, at bottom, we consider them unsuitable for this site.
But we also reject a number of questions that cannot be thoroughly answered without considerable research or special insight into English (such as questions about the origins of idioms, slang words, proverbs, or particular meanings of standard words)—on the grounds that the person asking them didn't demonstrate having done any research before posting the question at English Language & Usage. This, it seems to me, imposes a completely unnecessary limitation on our ability (or willingness) to provide detailed and useful answers to difficult and interesting questions of potentially broad and continuing interest.
EL&U has the potential to evolve into a free, accessible, Internet-era version of the great British periodical Notes and Queries (which still exists, by the way), but doing so would require us to adopt a more sensible and accurate approach to distinguishing between good questions and bad ones. The starting point for this would be to recognize that what makes a question bad is its excessive specificity (which makes it uninteresting to anyone other than the person who asks it) or its ease of being answered satisfactorily by a general-reference resource (since we don't aim to reproduce the contents of a standard English dictionary), whereas what makes a question good is its enduring interest, and its resistance to being resolved by a quick, easy answer.
Although the goal of building "a library of detailed answers to every [good] question about English language and usage" may not be realistic, I still find it appealing; and I would like to see this site continue to move, in its shambling, fitful way, toward fulfilling it.