I don't think discuss.flarum.org needs tags enabled for anything right now besides testing. So if possible, only Sandbox should have tags enabled, where users could tag like crazy. And I agree with @wizgriz that parent categories should be limited to one per post.
This topic for instance should only be in Meta. It's primarily concerning this forum, discuss.flarum.org, not Flarum's tag features.
"General Discussion" categories are dangerous, especially if you allow multiple categories, since everything fits in there. So when you're proposing a UX change that would obviously belong in UX, you start thinking "well, this is definitely of interest to the people discussing Flarum in general as well; I'll put it there too!". I like Discourse's "Uncategorised" category, because it implies a subtle slap on the wrist: "Are you SURE this doesn't fit into one of the designated categories? Well, no category for you then!"
We've touched on this before. Tags (and as an extension, humans) can not be depended on for cataloguing content. It's subjective and hugely prone to failure. Tags should be used to manage special, easily maintainable lists (i.e. 1-3 people share responsibility for a tag). I believe it's for that exact reason that GitHub will not let an untrusted user select any meta-data for a new Issue. The cataloguing is left entirely up to the developers - the trusted users - because they're the ones who are using this meta-data to try make their Issues queue match the way they've sorted things in their own head.
p.s. Sandbox having the same color as tags is kind of confusing. It looks as if the tags are children of Sandbox, which they're not.