The reason I ask this is because an answer I gave to a question quoted an Urban Dictionary definition and received 2 downvotes without any explanation as to why. Could anyone please tell me what (if anything) is wrong with my answer?
UD is a source that is written by amateurs, and rated by amateurs. I've used it from time to time, but it's best when supplemented with other examples.
As for whether or not the UD is an "acceptable" source, that depends on the nature of the question. For example, suppose the O.P. is asking something like:
What's a good slang term for a person who thinks too highly of himself?
I'm already familiar with terms like braggart, but I'm looking for something more modern
and slang – something that might not have made its way into a dictionary yet.
In that case, I'd say the UD is fine, particularly if someone is wanting to suggest a term that might not be widely known. However, if the O.P.'s question reads:
I'm looking for a clinical term for a person who thinks too highly of himself.
Then the UD probably isn't a good reference to substantiate the answer.
Going back your reference, if the only place I can find the term Question-Statement is in the Urban Dictionary, perhaps it's not good to use that as an answer to the O.P.'s question. However, if you can find and cite other published instances where that same phrase is used, then maybe the UD's definition, coupled with the other references you've found, could make a good answer.
As it stands now, it seems like the O.P. is perhaps looking for a literary or grammatical term, and all your answer does is suggest something from the UD – something that has a rather "amateurish" feel. It might have gotten upvotes as a comment, but, in its current form, it was vulnerable to downvotes as an answer. In other words, I don't think the UD reference is the sole cause of the downvotes, but it's the UD reference absent any other supporting research for this particular question.
(Not my downvotes, btw. Just giving my opinion.)
Urban Dictionary can be a resource for slang phrases that are not in traditional dictionaries. However, it is not at all an authoritative source, since it is mainly an entertainment site. For questions not specifically about slang or obscure phrases, if you can find another source, it would be better to do so.
UD is a great source. A quick scan of the definitions give you a reliable definition of the term. Moreover, it also provides variant definitions, and the up/downvotes give you an idea of how many people would accept those definitions. Note the wisdom of crowds. The crowd is rarely wrong. (E.g. The audience on Millionaire has been right 95% of the time - much more than the experts are).
I am very interested in the meanings and origins of words and phrases—and yet I think that I've used Urban Dictionary as a reference source in my answers exactly once, in response to the question What does “fleek” mean and when was it first used? And even then, I was more interested in investigating the theory that an Urban Dictionary user was the original source of the phrase "on fleek" than in relying on its multitude of arbitrary, sometime-compatible/sometimes-contradictory definitions for my understanding of the phrase.
That experience left me with the strong impression that Urban Dictionary is a very unreliable source of accurate real-world definitions, in addition to offering virtually no practical insight into how the meanings of words and phrases have evolved over time.
It's not the site's fault, really. The fundamental problem involves the large number of insincere, prank-oriented, adolescent-humor-driven users who submit definitions. The sheer volume of utterly worthless dross at Urban Dictionary is astonishing—and that's before you factor in the sincere entries written by people who can scarcely write a meaningful sentence when they try. Ultimately, there is so much random noise at Urban Dictionary that trying to extract a useful definition there can be more arduous and less rewarding than visiting a handful of online publications that happen to use a particular slang term and trying to glean its meaning there.
Urban Dictionary is like Wikipedia - super cool for certain words or certain kinds of research, but very unreliable and flaky. Some of the definitions are brilliant and witty, others utterly stupid. Political terms are especially thorny; they're a great way to separate conservative visitors from liberals. ;)
Urban Dictionary should generally be used in conjunction with other resources. I like to make comparisons between Urban Dictionary and Merriam-Webster, for example, following that up by typing a word into Google just to see what else I can find.