If everything goes well, your forum will grow and you will get more and more threads in each forum every day.
This becomes a problem, however, when there are so many threads that new ones are bumped off the first page in just a few hours.
So what do you do? You either split the forum into two, or create a subforum. We have had to do this a lot, and it's the natural way a forum grows.
The problem is that your members, having been used to seeing all the threads from your single forum, now complain about having to surf two forums. Sometimes threads don't get seen by members who would otherwise have seen them.
We had this problem when we created a Photo Sharing forum under our Digital Photography forum when it got too busy.
I propose having a Digest button/link in the title bar of every forum which has subforums.
When you click the link, you are shown all the threads from the current forum and all its subforums.
It's not quite the same as a search because it would use the normal template for showing threads rather than the search results template.
Another application would be in our classifieds forums. We had to split our classifieds forums up into multiple sub forums based on the type of products being sold. Now there is no easy way to browse all the classifieds quickly without doing a search, and no way at all to view them using the 'forumdisplay' template.
So I respectfully suggest a Digest button to present users with a display of all the threads in the current forum and its subforums using the standard template. Please like this post if you agree.