Edit: Due to the new banner shown to new users, all sites now have an accurate search engine description.
Unfortunately, it is the only site that has such a description. Not even the other trilogy sites (SF,SU) have one, let alone SE 2.0 sites.
First, why does Stack Overflow rely on a DMOZ entry instead of, say, a meta tag? And why is it the only site to merit a meaningful description? The others just pull up the question that was on top when the search engine most recently indexed it.
(Anticipating an answer saying: "It would require writing up a description for 99+ sites". But even a static SE network description would be an improvement.)