Proactive moderation is an implementation of writing standards for a community based website. It is used by H2G2. First, I'm going to explain it, and afterwards I'll complain about it (since that what seems somewhat popular around here).
The moderation scheme previously used by H2G2 was fairly straightforward. Editors and users would read through entries and if "something bad" was found, a few things could happen. If a user found an offensive entry, he could communicate to an H2G2 Editor (through a forum) that the entry was offensive. The editor would review it, and take the necessary damage control steps, either removing the entry (in the worst case) or leaving messages for the author. This scheme was kinda like e2 in the sense that only editors could kill entries, but they did so based on users' complaints.
The proactive moderation scheme was enacted when H2G2 was bought out by the BBC. I assume the BBC was worried about libelous entries or non-P.C. statements attributed to their name. Thus, they created this stronger system of editorial control. In this system any changes to the database (including new entries, edited entries, name changes and forum posts) are logged in a queue. Editors spool entries off this queue and review them. Entries/posts/names that don't fit the quality standards (called the "House Rules") are killed and an e-mail is sent to the user.
It's easy to break the House Rules if you haven't read them. For instance, there is a provision against trolling. If you post something troll-like, it will be deleted. As well, you can't give unsupported statements relating to a person or company (i.e. "Microsoft sucks"). You can't advertise a company or service, be it your own or someone else's. You can't post an URL in the forums, and only sometimes in entries. Most of these rules make sense, but the enforcement is unbelievably strict.
Now the problem with all of this is that you completely lose any sense of ownership. When I node on e2, I believe that my writing is my own property. By posting it, however, I am giving it to the community to be reviewed and either praised or nuked. I used to believe the same thing about H2G2 but with proactive moderation it felt as if the community was only secondary to the corporate brand of the BBC Online ConHugeCo. My writing is still mine (according to intellectual property law) but my identity in H2G2 is not. I don't feel like I'm a part of anything so there's no longer a reason to contribute. And that's why I left.
The failure of proactive moderation shows that large corporations are usually too anal to support an online community, and that the success of that community is heavily dependant on the freedom given to its users. If the site is too restricting why should users go there?