It's a total mystery to me which threads I am "subscribed to" and which not. (Relevant if you want to receive notifications.) Could we have a manual button "subscribe to this thread" instead of some automatic that seems to be in place now? (What actually is the logic of this at the moment?)
To view all forums you're subscribed to, go to your profile -> settings -> subscriptions (3rd tab) to see the forums you've manually subscribed to.Threads you're automatically subscribed to are the ones you open yourself or when people tag or reply to you.
If you want to manually subscribe to someone else's thread, go into the forum post, click "More" and hit "Turn Reply notifications on".
Hope that answers all your questions and solves the mystery behind it!