Also this shows again how badly we need micro-transactions i.e. something way less than 0.01 BTC. In fact, this would be great for any forum system, even not related to bitcoin.
Great idea. Donations are a more meaningful metric of authority than quantity of posts. The challenge is to get people donating more, by (a) making it easy, (b) allowing subdivision payments.
Perhaps subdivision payments are best handled outside of the bitcoin network by a third-party, like mybitcoin or a dedicated micro-tipping service.
This could be combined with a click-able widget which automatically donated a default amount in the context of the original page.
Seems like an obvious use for bitcoin.