I agree that the nation threads seem to be a net negative. They'd be fine if they stuck only to discussions that aren't a big enough deal to warrant their own thread. But, as it is, most major pieces of platform specific news get a bunch of their discussion inside of the nation threads. The problem is that it is sometimes difficult to have a good discussion about a specific topic in those threads, because there are multiple topics running at once. But, because there is discussion about a given topic happening in there, it likely leads to less interaction in the topic specific thread.
I think the nation threads are great for the relatively small number of users that spend a lot of time in them. But, for the rest of us, and certainly for any newcomers, they're sometimes not the greatest places to jump into a discussion.
At least, that's how it feels to me.