Those all strike me as very much advanced-user features, so as much as I'd enjoy using some of them, I doubt they'd make much difference to general-audience gro

Those all strike me as very much advanced-user features, so as much as I'd enjoy using some of them, I doubt they'd make much difference to general-audience growth. And I think you're being unfair about the spam/scam replies. Twitter has made great improvement there in terms of downranking/hiding junk replies. It'll never be perfect, but it's at the very least much better than it was. I have to go a long way down in most threads I look at to see that stuff if it appears at all.