I have noticed that even when I ask for immediate notification for a thread, I still am sent a generic email with a generic title to the email.
If someone wants immediate (i.e. non-digest) email updates for a thread, please include the title of the thread as the title of the email. I don't want 20 emails in my inbox which I must open in order to find out what they are about.
Also, the emails should have the same threadID so that email clients can sort them into a thread so that updates can be seen in context in one's inbox.