The thing is, you assume everyone who is new here has nothing worthwhile to say.

No, I assume that I don't want to hear them until they've settled in. You (and everyone else new) keeps assuming that you'll be globally silenced, rather than selectively ignored. You do understand the difference, correct?

New users don't only come from reddit when reddit behaves in a way its users don't like.

This is a largely false statement, user-for-nineteen-hours. New users come almost entirely from Reddit. They always have. And they always come when Redditors are mad. Thus, the influx of mad Redditors.

They might stumble upon hubski another way.

That happened exactly once. We gained about 50 people.

And global has been far from useless. I have found many thing that are not new user or reddit related there.

Usually, however, that's all there is. Besides, what do you care what I ignore? I'm not forcing my tastes on you in any way, shape or form.

It's not an inherently bad idea, but it has its problems.

You have enumerated none.

And having a feature that make new users invisible to old ones will have an effect on the new users experience.

How?

They won't get to interact with those who have ignored all new users - even if they have something worthwhile to say.

Try this:

1) Ignore me.

2) Click on a few posts.

3) Find me.

4) Interact with me.

5) Report back for science as to how difficult it actually is to interact with someone ignored.