I'm not sure about the technical feasibility of this, but I would love the ability to treat all messages in threads as if the user had selected 'broadcast reply'/'also send to #channel' so I don't have to ever open threads.

Not a bad idea, but requires a bit of work. But it's definitely doable with the systems I've already programmed. The hard part is storing the preferences for it per-channel, and how to show it in the UI. I think I will need to make the Slack toolbar bigger if these kinds of options are to be implemented. Not sure.

Thanks for considering the request - it would be a killer feature for me and the many Slack thread haters that I meet :)

Might be worth opening another ticket for this, but unless I'm missing something there's no way to enable 'broadcast reply' when replying to an existing thread, only when starting a new thread.

It's a button in the toolbar. It's also in the menu. I don't know where else to put it to make it more visible. The design of the threads feature in Slack is weird, and I don't know what to do about this. it's also weird in Slack's own client. :/

It turns out this is impossible with the API that Slack has. There is no way to expand every thread in the channel because there's no way to reliably know when threads are being replied to, unless you subscribe to ("follow") those threads -- and there's a limit to how many threads can be subscribed to (though I'm not sure exactly what it is.)