Wha??? Windows, me??? No reason to get nasty!
But really, thanks LOTR, that helps some. I've been a good LW sharer, Mac only, for years. Yes I can upload, though apparently it's pretty rare that anybody actually wants any of my stuff.

I see that I do have UPnP enabled.

I'm already using my junk filter same way as you describe. Seems to work OK. It's not very smart but I wouldn't call it a dumb-*** either. BTW I'm hesitant to block hosts; I don't suppose you know any way to tell the difference between hosts that are spammers only, and hosts who are legit citizens who happen to have dl'd a spam file and are inadvertantly sharing it?

I didn't know "many OSX users don't even bother with their firewall being on", though I am familiar w/ the conventional wisdom that Macs are safe from bad guys. I just tend to play it safe when swimming in unclear waters; you know what happens when one assumes...

Re your point that configuring firewall only allows LW access: are you saying bad guys--if there were any out there preying on macs--would not be able to use that access for their nefarious schemes?

What is NAT? Can you recommend a post or page that 'splains this stuff a little further--NAT, ports, forwarding, etc.--for dummies?