bbPress is starting to use AJAX for updating various content, which allows us to avoid hard page refreshes.  The benefits of this are two fold.  First, we only have to update part of a given page instead of the whole thing; we save time, database hits and bandwidth.  Second, the user experience is improved (or will be when we figure out the best interface for all the new shiny bits).

Everything should degrade nicely if the client does not support AJAX specifically or Javascript in general.