There's plenty of incentive for asking good questions already:
- Well asked questions are more likely to get good answers
- You might earn a little reputation from your peers
- If many people view the question, you can get a badge
- If you get enough up-votes, you can earn a badge
Stack Overflow doesn't have a problem with question quantity, more with quality. The number of questions posted per day is rising. Some say that the number of good questions per day is not rising at the same rate.