Found this in the "Why Doom10?" section:
http://doom10.org/index.php?topic=17.0
4. Outdated information
"The vast majority of the information on Doom9 was years out of date and it was often very difficult for newer members to distinguish still-valid guides from those that were half a decade outdated. Doom10 will focus on modern video technology."
I can see the point of this. Some of the forums and guides on Doom9 are really ancient. A fresh, modern reboot is needed once in a while.