I prefer grouping into forums on the home page. Grouping things into broad topic areas lets me zero in on the ones that interest me most rather than sorting through a tag cloud or list. If I'm not in the mood for political discussions, I can stay out of the Politics forum rather than trying to figure out which tags are "political".
On my board, we also use the forums to create areas for specific groups (e.g. many of our members belong to a specific Christian denomination, so we have a forum specific to them) or for topics that we want to set different rights or roles for. For instance, one of the reasons for putting that denomination in its own forum is so we can easily keep the non-members who occasionally harass them or post off-topic simply by removing them from the secondary group that controls access to that forum.
There's also the question of the quality of tagging. I'm pretty thorough with tags myself but I was also a librarian in a previous life so classifying stuff is part of my background. Most users, IME, either don't use them or don't use them well. Without decent tagging, sorting through a monolithic list like that becomes a challenge esp. if, as I suggest in my first paragraph, you're looking to avoid or focus specific areas of discussion.
That said, you could just have all your discussion in one big forum and only use other forums where you want the kind of segmentation I mention above. That would sort of accomplish what you want, wouldn't it?
All of which comes down to, I'd be fine to have it as an option but wouldn't want it as the "norm".
