@dave, I thought that commenting would automatically turn on the "watch" function, but it seems I have to actually press the "watch" button after commenting.

Is it possible to change this permanently somewhere in my preferences so that I'm "watching" every thread after I leave a comment?

Yes, go to your profile, click the "change your site preferences" hyperlink, and select "yes" in the Automatically watch threads you reply to field (the sixth one down).

That was easy!