Hi,
I'm not really trying to ctiticise "categories" - their application in the situation that Christof describes seems to be very appropriate i.e. A small number of categories and a fairly "tame" audience of failrly computer literate users who can be "encoiuraged" to comply with the system. I'm really explaining why we're not particularly interested. Our application is in the KM area and we'll have many many categories and sub categories and sub-sub categories etc. fairly non-computer literate users and no real way to police the input of categories. Hence the alternative approach.
If we add categories to forums I take it we could "switch off" that feature.