Flash has the advantage that, when used well, gives the user a more unique experience. Whether it is justified for use as the entire interface of a forum is still debatable and I have seen some fair efforts, yet nothing in the way of total innovation. Until someone can find a TECHNICAL advantage of using Flash over what is currently the most popular language when it come to forums — PHP.
However, the two can be combined and I have been mesmirised by a site by a guy calling himself
Sephiroth which is dedicated to exactly that.
I am planning my own site at the moment and will have around foru versions. The flagship site, which will cater for braodband users and very patient dialup users created entirely out of Flash components and PHP; a primary site that will mimic the look and share the content of the flasgship site, created using XHTML, PHP and CSS; a secondary site that will be just text for text only borwsers, only text heavy pages will be available; and one for mobiles that will have just the news section and certain articles. The site wil send each user to the appropriate section upon logging onto the root domain.
With a little work, anything can be possible, it just depends on where your VALUES lie. Mine lie in giving the best possible visual experience to anyone without them realising they may be missing something. Automation is the way forward on the web.