I'd disagree here, because I think this is not, how categories should be used.
First of all, there shouldn't be too many categories, because the UI becomes confusing, possibly problematic on mobile and it becomes IMO harder to find stuff. This can already happen sometimes at 3-5 categories, but it's probably going to be worse, the more categories we have. Also, categories can't really be removed, once added, hence I'd be careful with adding ones you don't really need. While it is possible to move posts between categories, it requires either database access or manually going to a special "tools" menu for every single post.
I see categories as a way to separate different types of content (not neccessarily post types). In my opinion, a Q&A "thread" is something different from a Challenge "thread" or a Blog or a Wiki. I'd also consider a Meta post something inherently different from a Q&A post. They serve different purposes to visitors of the site. Some inform more generally, some are for specific problems and others are a bit more fun. All are about the underlying community, but they address different interests of it.
On the other hand, bug reports are not something completely different in my opinion. As with feature requests and support questions, they are about "issues" or unclear stuff you came across when using the site.
In the same way, most [discussion]s we have here, don't really seem to me to be community-policy discussion (which should also be mostly in the per-site meta communities), but rather pre-request discussions about new features and "mistagged" support requests. Just very few are real discussions IMO, so it doesn't really make sense to separate them.
Furthermore, there are ways to group posts based on their content: tags. We have several nice features for them, including tag hierarchies, "topic" and "required" and "mod-only" tags. This is IMHO sufficient for grouping posts right now.