I think it would be easy to switch the color of the "new" icon that appears next to new posts from blue to red (like you suggested) for new posts that have been submitted in threads that haven't been posted in over a certain period of time (say, a month or two). Was that the kind of idea you had in mind? I think it would help a lot.
That would work too, except it might be perceived as a "CLICK ME, I'M RED!!"... I was thinking old timestamps displayed in red when viewing the thread - not clickable, but screams "PAY ATTENTION TO THIS DATE".