I am very interested in the same. I noticed on a vB forums that all the sub-topics are displayed so users can see them right away. The XF forum requires a user to click on the sub-forum link which I don't think is very intuitive for most users. Let's say you have a forum called "Bug Reports" with sub-forums for "Version 1.5, Version 1.6, Version 2.0". Many users will simply make posts in the main Bug Reports forum and not see the sub-forums.
I checked all 8 pages of Add-ons and didn't see anything pertaining to this matter. Does anyone know off hand which add-on might offer this change?