The site sent email when someone added a reply, but I'd be just as happy (maybe happier) to have a Watched Threads page along the lines you describe.
I have enough Spam in my eMail box already; Pills, Won somethin', phishers, asciis i can't even decipher... name it. That's the first thing i turn off on any forums i participate(d) to and if it doesn't stop within a week or less i simply cancel the accounts.
Watch threads... the idea is good - but, seriously i just have to compare two columns with last reply tags from me and others. Some left side Icons are on fire, blue cues, etc.
There's always room for improvement, but if i find something interesting what stops me from typing a reply to subscribe permanently as of now. Maybe personal "moderating lock" symbol on the left would indicate me i've just unsubscribe from it... but, that runs the risk of creating a groupies membership as i explained earlier. Gang X ignores Gang Y which in turn starts to ignore Gang Z of noobs which in turn decides to ignore Gang W in Off-Topic sections.
So much for a community sense.
Loneliness is a two edge sword and once someone realizes he is talkin' to the wind, he'll go away.
Don't worry, he may return to determine if a board shows some activity -- too late for Gang A or S or M to regret his absence.