Re: Categories or tags based navigation ?
It would work fine. 
I hacked mine so users would tick on tags that are pre-defined, using only one forum. I thought it would be more suited for the target users who might not be familiar with the whole concept of “tags”, and I think it is working as expected.
The way I could think of in how to enter tags in the first place is pretty silly but ah well…
You’d be letting users enter the tags themselves, so it’ll be logically nicer.
