Maybe I'm dreaming, but I think Show Unread Posts Since Last Visit only displays unread posts in the General Discussion forum. I clicked on the [NWC2] General Discussion and [NWC2] Instrument Trees links just now, and found messages that I'm pretty sure didn't show up when I clicked Show Unread Posts Since Last Visit.
If I'm right (and I might not be), then there's an improvement waiting to be made, or we all need to be aware of the limitation.