I probably never clicked “Watchlist” button, while I was frantically editing the “watchlist” list of threads, without noticing any effect in the “Show new replies to your posts” list.
As a Newbie, "Show new replies to your posts." works just the way you expect it to, while Watchlist doesn't do anything.
Maybe, since this thread is about improving things[/url], theymos' suggested tickbox "Automatically watch topics after replying to them." should be the default behavior for the forum
for new users.