In your theme settings there is one setting that controls this:
Default postlist format. Collapsed (only show main topic with total number of replies. Expanded (Show topics with a threaded list of all replies)
With that being said, the vast majority of sites keep this to collapsed since it really is easier to follow when it's collapsed. THis allows them to see the subject of each individual thread and then click on the thread to see the replies to the thread. Also, on busy/large forums expanding the threads on the postlist page can be much slower do to the number of posts it has to process.
