Dr. Guy, I am not aware of a simple way to watchlist forum threads. If it is also a blog article you can go to the persons blog and watchlist it there. There is the 'Join mailing list!' option at the bottom of the forum, it might be a watchlist type effect, but since that option is also on blog articles I doubt it would watchlist things. Looking at the watchlist icon on the blogs, in theory you could manually enter the article id for the forum thread and watchlist it that way. In other words: https://www.joeuser.com/action.asp?cmd=ADDWATCHLISTITEM&ACID=(insert your stardock id number here)&AID=(insert article number here)
Put in the numbers and that might work. Next question would likely be 'how do I find my stardock id number and an articles number?' Click on your name in a comment and that will take you to the JoeUser Profile, Stardock.net ID is the first thing listed. Article number is listed in the url (starts with http and usually displayed at the top of the screen) of the article/forum thread. It's the number after AID= and will be at the end of the url. If your on the forum there might also be a # with more numbers, ignore that part. I can't confirm this would work for forum threads, but I would expect it to.
Dharmagrl, it sounds like what you said would cause all of your articles to be watchlisted. From the way that option is worded it doesn't sound like it will watchlist other peoples forum threads. But I could be wrong...