There is a watchlist function, though it was temporarily disabled with the new forum code/design launched a couple of weeks ago. It should be back in time as the forums continue to be enhanced, though.

For now, you should be able to just reply to a thread with a 'tag' post though (if you don't have a useful reply to make that is) and it'll then be listed in the My Replies page so you don't lose ones you want to keep track of. Those threads that you started are listed in the My Posts page as well.