Then problems you are having with the 'Active Topics' may be to do with all the threads being moved over to the new site (so all have the same age if that makes sense).
New posts are signaled by the 'glowing' icon, next to the Topic on the main forum window. You may have noted a "Mark Forum Read" button at the bottom of the Topics List. When you press this, it will turn off all the 'glowing icons' until a new post is made within that topic.
If any of that made sense, then well done because I just read it back to myself and have established it's far to early for a Sunday.
EDITED TO ADD: There are 4 options of how to display the Threads and their respective Posts. This setting is in the User CP > Edit Options.
Here is a breakdown of what the 4 options do. Clicky!
Hope that is of some help.
Ross