That's a very good point. I'd be much happier if they said the new guidelines is no personal attacks. That's very clear and easy to check.
Instead we end up with this slippery slope of certain types of negativity vs. others and all it really means is the mods have a blank check to remove anything they don't like that isn't meaningless happy fun compliments.
As Sam pointed out, gratuitous negativity has always been against the guidelines. Personal attacks are, too, of course.
The way this stuff really works out is in practice, so judge it based on what you see actually happening on HN. The intention here is pretty modest: to nudge the community slightly in the hope that the culture will grow a little more aligned with the guidelines. We're not interested in false positivity either.
That doesn't mean this isn't a big deal. Gratuitous negativity is a hard problem because it's mostly unintentional. That's why the most important part of this is asking the community to gently give commenters feedback when they see it. Any significant effect of this guideline change will not be in what moderators do, but in what the community does. Given the quantity of comments posted to HN, there aren't enough moderators to make that big a dent by direct intervention anyhow. We like it that way.
Instead we end up with this slippery slope of certain types of negativity vs. others and all it really means is the mods have a blank check to remove anything they don't like that isn't meaningless happy fun compliments.