|Welcome to the Monastery|
> some people do reply to mails by composing a new message
Perhaps the ability to manually link a message to an existing thread would fix this problem?
> a way to specify an auxiliary sort
Great idea. Why don't the other webmail systems have this yet?
> set sort=threads and set sort_aux=last-date-received
Maybe provide macro expressions for 31337 h@x0rz to use?
> fold/unfold (or open/close, show/hide... whatever) individual threads
> decent response times
No ads = no need to guarantee slow response times.
> What, too difficult to implement?
I think I can do all this and more in about 11 lines of Perl.