Noticing that people replying to other people I don't follow are showing up in my local timeline. So now I'm worrying: every time I have a conversation with someone, am I polluting everyone else's timeline with an unending series of replies?

That was one thing "the bird site" got right: you didn't see peoples' replies to people unless you followed both of them.

@csilverman Amarok gets this right. Replies are unlisted by default. I think it's just the web interface that defaults to public for replies.
@pete @csilverman The usual opinion on this here is that this is a good thing, as it makes it easier to jump in to other people's discussions and thus encourages conversation.

@lambadalambda @pete I'd generally agree, although I'm still trying to get a sense for how accepted that is on here. I've replied to a few people and gotten sort of a "wtf where did you come from" reaction—maybe some people aren't used to that, or don't really like it.

Which is why I was worrying that other people were seeing their timelines fill up with my replies and thinking "jesus, asshole, shut the hell up and let someone else talk for a change".