"How about have a drop-down list of topics/subtopics next to the submit button when creating a question and only enable the submit button when a topic/subtopic has been selected."
We have this on the homepage. I think it might actually cause more mistakes than accurate selections. I'm also not sure I want to add another hurdle to jump just to post a question.
"At least the questioner might think about which topic is appropriate, although they might just be lazy and simply go for whichever topic happens to be at the top of the list."
The latter is likely, but not out of laziness, they might just not know which sub-section is best.