I'm talking more than message threading here, but threading all messages in a conversation, with your replies, and read messages minimised while unread are expanded. Some kind of quick reply box at the bottom would be awesome too.

Right now the "Conversations" add-on is the only thing Keeping me on Thunderbird.

Take a look here https://bugs.kde.org/show_bug.cgi?id=96968

Thanks. I'm subscribed to that bug, now. Hopefully something will come of it, especially since the Mozilla Foundation just announced that they're not supporting Thunderbird any more, meaning that we may soon have NO FOSS option for reading emails in a conversation view like Gmaill...

a simple solution for a start would be a clickable "replied"-icon:
viewtopic.php?f=83&t=106929&p=247068

I think this is e real "must have" to a modern email-client

For the record, I've recently started using KMail and I like it very much, but this is one of the few things I really miss. The current tree-like conversation view is nice for conversations with three, four, five replies, but if a thread has twenty mails (quite often the case on mailing lists), it becomes bad.

Cheers,
Sven