The quality of questions is the main problem, yes. May be they can be upvoted or downvoted, so they would appear more or less frequently?
Or, and while we're at it — location filters for location-based questions would be the greatest (although much more hard to implement) feature. I don't want to answer someone's question about his homework, clearly directed for his friends, but I would gladly give advice to a tourist who found himself in a place I know.