I remember we have had this discussion before. Hopefully there weren't any good arguments why we shouldn't have the title of the thread as part of notification email subject but I can't think of any.
Most e-mail programs group the e-mails by subject (thread), having all the notification e-mails with the same subject line does not help much - I would say it actually defeats the purpose of the notification.
Would be too difficult to add the "thread title" to the notification email subject line? Is there any good reason not to do so?