You can read more about it here: http://www.vbulletin.org/forum/showthread.php?t=245839
Here it is in action on my vbulletin forum: http://www.the8thlegion.com/forums/15-The-Public-Forum
This is the most important modification on my board because I have lots of forum noobs who easily get lost on my vbulletin forum. Some are too lazy to view individual subforums even though there are pictures and descriptions that describe what the subforum is about. I basically get these members from sites like Facebook and Twitter where there are no subforums. There is mostly only one feed. That's what makes the All Topics modification so important.
I personally began to like it as time went on because it became easier to moderate the site and can now fully appreciate the usefulness of this mod. When members create threads in the master forum I designate that would belong to another subforum due to it's topic, I can easily move it. Because All Topics displays what subforum the thread came from in the master forum list, members can see that their thread has been moved. Eventually, those members then start creating threads in the correct subforum.
All Topics has been an effective learning tool in guiding noobs into posting topics into the correct subforum. Posting topics in the wrong subforum is the largest pet peeve I've ever had as an admin. I would tell them in the thread, send them a message, ask them nicely, politely, warn them etc. It works some of the times but after I installed All Topics I haven't had to do so since. It's strange how quick people learn stuff after only guiding them instead of telling them.
This modification would be the most important step towards attracting a facebook-like crowd.