Technovia has an interesting article "Why Digg is doomed, and why the wisdom of crowds is mob rule"
"... the fundamental problem with the whole 'wisdom of crowds' approach. It makes the assumption that online debate is largely rational, when - in fact - an awful lot of it is simply emotional. There is very little quality debate on the open internet on any topic that attracts a large level of attention. Yes, there are small communities that remain coherant and reasoned - but these are rare."