To add to the given answers, and respond to some comments on the original sharia-investing question and this meta question:
From my perspective, what matters is what the central intent of the question is. If the question is "How can I invest in accordance with [insert religious/ethical/moral/whimsical restriction here]", it is on topic as long as the restriction can be communicated clearly enough that answers can address it. Even if the restriction is quite broad, if it is possible to give a good answer, we should leave the question open.
We have seen in the past that even seemingly broad questions like "How to invest ethically" can usefully be answered with pointers to "socially responsible investing" funds or the like. Of course, such answers necessarily come with the qualification (explicit or implicit) that the questioner must use them as a starting point for research to see how well the options mentioned (e.g., SRI funds) match what he or she has in mind.
(Incidentally, in this regard I think questions about Islamic banking/investment are some of the least objectionable, because there are many investment vehicles specifically designed to meet that criteria. It's much easier to answer a "broad" question about Islamic investment than to answer a "specific" question like "What investments will help save the Devil's Hole pupfish".)
To my mind, where questions would cross the line is if they are about the nature of the restriction itself, rather than about how to invest in accordance with it. For instance, a question like "Are REITs compatible with sharia investing" would probably not be on-topic. Such a question is not asking how to invest, it is asking about details of sharia.
More generally, this site is not just for open-and-shut factual questions like "What is the IRA contribution limit". A vast number of questions here require answers that are opinion-based, because, for many issues in personal finance, there is genuine debate and idifference of opinion. This is different from other StackExchange sites like StackOverflow, but that's not a bad thing. Rather than closing questions because they might be answered in a misleading way, we should try to develop a culture of careful answers -- that is, answers which, when necessary, rely on opinions, but distinguish those opinions from facts and, as much as possible, back them up with reputable sources to make clear that the opinions have some currency in the wider world beyond the mind of the individual answering.
Or, in short: a question may have zero right answers, one right answer, or many right answers. But every question has many wrong answers. We should only close questions that have zero right answers. We should keep questions that have many right answers, along with those that have one right answer. It doesn't matter how many possible wrong answers there are; that needs to be handled by downvoting (and if necessary deleting) incorrect answers, not by closing questions.