I guess the only problem there is that it makes it harder for one community focused on a certain topic to "branch" to another when things get rough, whereas now

I guess the only problem there is that it makes it harder for one community focused on a certain topic to "branch" to another when things get rough, whereas now you can just start "/r/ruby_two" or whatever if /r/ruby makes you mad. It's definitely a trade-off, but I think Hacker News is still high-quality enough that tagging would work.