I think the current NEW icons are fine. Note: do we really need the 24 hours icon? I mean a quick glance at the dates of the posts (which are already sorted by most recent date) shows when the last post was made.
I understand that emailing anyone subscribed to a thread whenever a new post is made can be very taxing to your server. How about a page we can visit which just lists the threads we're subscribed to if there have been any new posts? Call it "New Posts in Subscribed Threads" or something. Something folks can go to and instantly see what WOULD have been emailed to them under other forum systems. Just an idea.
I do have a single suggestion that I think you'll agree is a good idea :). You automatically convert any URL to a link, but you don't change the target. You should, so that the links open in new windows.
Randy
www.RRDeals.com