Answers should explain why they are correct. "Back it up" is a vague concept, but in my mind it means an answer has to have an explanation of why it's correct.
You cannot "copy/paste" the answer here, hit compile, and see what happens, and if it doesn't work try something else. Nearly every question asked here has answers which you get to try - once. Many of the questions have significant impact on the life of the person asking. And the reality is that nearly every question here has details we do not know from the asker.
For this reason, I think it is critical that answers have an explanation for why they are correct. Without this explanation the asker cannot take answers, understand the motivations/reasoning, and then apply it to their situation.
Sure, we can say "reader beware" and not care as a site about whether or not people get meaningful answers to questions here. But I don't feel this is a good idea at all.
People come here for solutions to their problems. The problem with answers not explaining why they are correct is they only give an answer, not a solution.
Additionally, the primary audience of Stack Exchange really isn't the asker of the question. It's providing an online resource that is superior to forums, discussion boards, and other online resources. If The Workplace just becomes a discussion board where everyone places their opinions, it will become noise and much less useful.
I really wish everyone would take this more seriously. It doesn't mean having long answers (though longer answers can have more context than shorter ones) is a requirement. If you've been around here much, you'll know that I am very selective about the types of questions I answer and generally write "long" answers.
A good example I just saw on a recent question :
You can get anyone to write you a recommendation letter, but I can't see an employer taking one from another student seriously. It is irrelevant to their needs and a student would not be thought a reliable, informed, and mature reference.
The very fact that you thought it appropriate would work against you with all interviewers I am familiar with.
The bolded is the "why" for the first part. It could probably benefit from a longer "why won't they take that reference seriously?" explanation but still gives "why." The second paragraph is a personal anecdote, but that personal anecdote is not the primary justification for "why is this answer correct?"
I won't call out an answer that fails this, but there definitely are many here.