The content of developer network would have some pages something similar to what appears on the Fedora, Ubuntu and ArchLinux wikis to some extent?
The benefits of a developer network wiki have to be outlined clearly. With github for example, the benefits are very clear: it's easy to put your code there and to contribute to other people's code *and* to their documentation. With the Mozilla Dev Network the benefits are also clear; more docs on common web stuff leads to quicker/easier development with all the gotchas and hacks and workarounds documented.
There are other projects like OpenHatch that have tried to encourage involvement in the development of free/open source software and they haven't gotten much uptake. They just didn't provide enough useful resources though some of their tutorials were great: https://openhatch.org/missions/