no, that would be "hangi soru?" or maybe "ne sorusu?" depending on the context.
It is not a direct object in this questions and doesn't need to be in accusative case.
"is" is the verb in this sentence. Then you ask "what is?" and the answer is the question.
totally agree with dragan17a's answer, but a more grammar based one is like this: "is" is a copula, a grammatical equals sign. because "what" and "question" are the same thing, "question" is kept in the nominative.
What if I wanted to say "What is his question?" ? Would it be correct to say "Sorusu nedir/ne?" or do i need Onun?
Where on the web can I find a really indepth explanation of -dir? I thought I understood it, until I saw it in a question.
Just read the comment of Orde90 and Ladynurington at the beginning of this discussion.
the n on the end of soru makes it "your" so it becomes a very loose term soru nedir " what's the problem" to "what is your problem?" (or question) the dir on the end of words emphases it as sort of a fact or definite statement. For example: If Turkish people had the saying, "that is the last straw!" it would sound like, son pipetDIR. It is just emphasizing a statement, that we do with stress of our words, such as Americans always use stress of words instead of actually having a word for it. For example, to define what you are asking a question about in Turkish, depends on the placement of mi? If I ask, "Is THAT GUY from Italy? It would be written as O adam mi italyali? But if I ask, "Is that guy from ITALY?" The question would be written as, O adam italyali mi?