Thankfully Drupal gives us a lot of leverage, so implementing tagging and fields and such shouldn't really be too difficult. I won't know for sure until I look into it (there are always gotchas), but that's not the main type of complexity that worries me. Implementation complexity is hidden from users, so it's just the webmaster's problem. But interface complexity, that can be a problem for users (e.g., if it's intimidating). So that's a more serious kind of complexity in my book.

As you suggested hiding advanced stuff inside a collapsible interface is one way around that though. On the other hand I worry that these sorts of advanced bells and whistles woill add to the overhead involved in forum maintenance. It's a bit counterintuitive, but often simpler, imperfect systems that don't need too much hand holding work better in practice than a system that is "well engineered", at least on paper. Think wiki vs a more traditional document management system.

None of the above constitutes an opinion, just random considerations. I'll think about all of this in more detail when I get to it.