That would be handy. As an interim measure I have noticed that the new threads are marked with a little red circle when you go into a story so that may help.
Check out the "story map" for the most recently updated story.
Notice the black "circles" next to every thread. If the story was updated in the past few hours (no, I don't know more about the time limit), that circle will be red.
In fact, it seems like it starts as a red dot ("full" circle). Then, it becomes a red circle. Then, a black dot, and finally a black circle.
I think it'd be good enough to identify new threads, but they should make them last longer. 48 hours between "red dot" and "black circle" sounds about right, IMHO.