That notification was actually what slowly made me an active Twitter user, because they kept surfacing me interesting content and giving me a reason to come back and it became a gateway for me to discover other great content on Twitter
I like HN. But I never look at particular users' posts. I just look at the front page, or sometimes at the comments list, so see what people are talking about.
I think for it to work people would have to start blocking more and using more mute words.
The thing where someone retweets something horrible because they disagree with it is so wrong and is what is being badly gamed.
It's funny. For all that I support free speech, I don't have the patience to read much of it. So I end up gravitating to well moderated forums.
Bundle it under "experimental" or "advanced features" in an easy to use page with a very clear explanation of what each feature does.
I think opt-out makes sense though, for users who don't a feature.
Features that will go viral are worth adding for all, features that did not can be kept there.
Twitter very likely did market research on its effectiveness for the target market they were trying to please. Probably that's just not you.