I think Tags is a great things for forum
with massive and various information in a forum, tags can help users organizing these information because tags can classify subdivisions than categories. also user can click tags in home page so that they can get their interested topics directly.
there is a tags based forums in china:v2ex, i think its a excellent forum
This has been asked many times before. There is an open issue . I read a bit of it and it seems I wanted it to be a plugin, but I'm not so sure now. Maybe categories can be reworked to behave more like tags