+1 for a conversation mode
Note that historically a "conversation mode" and the traditional "threaded view" are different things - a threaded message list generally only groups related messages from one mailbox together. Conversation mode groups messages from all mailboxes together. The difference is that when using threaded mode for example on your inbox, it will only show other people's replies in the thread, whereas in conversation mode you will also see your messages (from your sent mail) in the thread.
Conversation mode is vastly more useful than the traditional threaded view, but it can be computationally more intensive since to access all your mailboxes (inbox, sent and archives, at least) for it to work properly.
(I'd totally pay for this feature!)