It's my preferences that forums be flat.  That is, one continuous stream of comments, flowing like a discussion, each person contributing to the last thing said.  For those times when a user must reply to something somebody said five posts prior, he uses the "quote" option.  That's my opinion.

However, some people like nested forums.  In a nested forum, the level and position of your comment's placement depends on where you click "reply".  This results in a tree-like forum where not all recent forum posts exist at the bottom.  For example: