The default for visitors (with no account) is to show one post at a time, with all the message titles for the thread shown below. When I changed my setting for "Message reading display options" to "Default", it reverts to this behavior. However, the fact that it has "Default" as an option (in addition to the specific options, one of which is chosen when you select "Default") at least implies that there should be some way somewhere for the webmaster to define what "Default" means.
