I would suggest putting the thread title in subscription email subject lines, so people know which one of their threads were updated without opening the email.

Also, it would be good especially for Gmail, since right now, without any differences in the thread title, the subscription emails are ALL in one email, so a person would have to sift through every single email to see which one they haven't checked yet.

I'd like to second this. Some topics are more important/urgent than others, so it would be great to see which thread got replied to. I'd replace "Topic Subscription Reply Notification" with "New Replies in <#TOPIC_TITLE#>". And why not throw in "Private Message from <#USER_NAME#>", while you're at it.

Not that I use subscriptions, but I know my members do ... this is a good idea.

Keith