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 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.