My criteria for nutrition related question is simple: Are answers required to pass judgement on what is considered healthy, or what constitutes good nutrition?
There are lots of diet theories out there, and all are controversial. There is research disputing even the most popular of them. The world is full of people who believe to have found the key to long, healthy life, and nobody can prove it. If we allowed such discussion here, we would gain nothing but flamewars.
So, imagine that somebody creates the theory that eating food which has been heated for more than 30 minutes is terribly unhealthy. (I made that up on the spot, but won't be surprised if someone somewhere has already come up with it and follows it to the letter). If anybody comes along and asks "Is it true that eating food heated for 30 minutes is unhealthy", then this is a discussion we don't need. We cannot even define what "healthy" means. (What if there is a diet which reduces your Alzheimer risk but increases your heart attack risk? What about a diet which gives you 10 years longer life but also gives you a depression? Are these healthy or not?). If somebody insists that all of us would live 10% longer if we didn't heat our food, there is nothing we can do to prove that he is wrong or right. It is the worst kind of subjective question.
On the other hand, imagine a person who has built their own, subjective opinion on the subject, and asks a question with this opinion as a background. Let's take again my hypothetical "up to 30 minutes of heat is OK" guy. If he comes along and says "I don't eat food which has been heated for over 30 minutes, but I love bread. Is bread typically baked for less than 30 minutes?", then we can help him. The answer only requires knowledge about how long bread is heated, which we can easily provide. If he asks how to change a given recipe to be prepared in less than 30 minutes, then this is again a good question. The reason may be his personal health beliefs; as long as we don't have to confirm or refute the health/nutrition beliefs of the OP, but can just focus on answering how to choose or prepare food which conforms with such well-defined beliefs, the question is OK. I say well-defined, because a question like "I want a list of the names of all natural sweeteners" is not answerable, as there is no good definition of "natural" (and some of the best candidates for definition would put HFCS in the natural group, which would horrify many health-conscious eaters).
By this criterium, the question you linked is well defined, at least in its current state. The OP does not ask whether he should avoid carbohydrates; he made this decision independently of us. He is just asking which milk has the least carbohydrates. This question is not subjective, and not likely to end with a flame war, so I would leave it as it is.
In a combination with the fact that we as cooks cannot tell you how food is digested (for example, we can tell you that there is lots of Vitamin C in spinach, but not whether it is bioavailable, so this is a plain off-topic close), we have a set of good criteria. When we apply them to the most types of questions we get in this field, we arrive at roughly the list from Aaronut's answer.
