currybetdotnet has a beautiful post on the differences between h2g2 and Wikipedia which I strongly recommend you read, and which includes this typically astute point:
h2g2 offers more of a closed model, with an enforced registration system before users can participate, but even so Wikipedia is an interesting contrast to the BBC approach to community spaces online. Over the last couple of years in our social spaces we have only gradually been able to make a move towards firstly the post-moderated model, then to the reactively moderated model. h2g2 was one of the first (if not the first – my memory doesn’t serve me so well here) BBC online social space to have a community trusted with reactive moderation.
He also points out the essential copyright difference between the two services: on Wikipedia, nothing is “owned” by an original author, whereas on h2g2 the original content is very much owned by the original author. In that, it’s fascinating to see how the two things have evolved differently; and important to point out that both need preserving. Wikipedia doesn’t replace the need for h2g2.