This hack will allow users to subscribe to receive email notifications when new replies are made, even if they did not post to the thread.

My opinion: this hack will drammatically increase users activity

No offense but that would really suck having an email sent for every reply even if you did not reply, if you have a large site you would have thousands of useless email. I am sure it can be done though with the proper implementation.

thousands of email ???
Well, you will recieve email only if somebody reply to specific thread, not from whole BB!!! It is mean, that you will recieve as much emails as number of replyes in this thread.
Also users suppose to have an option to unsubscribe later.

Actualy, this hack already implemented in vBulletin.

this would put a HUGE strain on your servers email capabilities!

True. But I have unlimited sendmail (well, I know - nothing in this world is free or unlimited )
And I willing to put some strain on my server if users will come back more often....

huge is an overtaking term. It wouldn't be unbearable nor would it be deadly. It just would slow things down and cause some strain.

I have no idea why you would want to do this in the first place...

If you're just talking about allowing e-mail notifications for replies in addition to new topics, which I assume you are, you should check out the hack I did for the UBB5 series. It has a limit on the number of e-mails that will be sent out per thread, so things don't get too out of hand. I use this on my board, which is just shy of 17,000 members, many of which use this feature, and haven't noticed any slowdown. I have a version of this hack working on my UBB6 test board as well, as will release it once UBB6 is out of beta.

Cool Leshrac I will be looking for that one.