>>5
It does fit with the definition of "forum":
* 1. forum -- (a public meeting or assembly for open discussion)
* 2. forum, assembly, meeting place -- (a public facility to meet for open discussion)
though, imageboard/textboard purists would still disagree with you, since most people who think of a "forum" think of forum software and a layout like vBulletin or phpBB.
Also, saying that it's "anonymous" is a bit of a misnomer, as people can post with names/trips, even though it is discouraged and anonymous posting is the norm. AFAIK, /book/ is the only world4ch/4chan board that has no name text field.