Limitations in predictions market are well documented (and include Muhammad's points above), and constrain their practical application to a well-defined number of situation.
Crowdsourcing suffers from the same limitations, which is not a problem, as long as you limit its application correspondingly. The problem occur when you stretch it outside the required constraints and yet present the results as "scientific", i.e. as a good proxy for what the crowd thinks.
That's what professor Vassilis Kostakos's theory ultimately comes down to (or should - I don't know, I haven't read his report). Apps like Digg or Amazon's review are not scientific applications of crowdsourcing, and thus their results should not be seen as precise representation of our collective thinking.