I guess I never really looked at firewalls and NAT as issues. I can see how it might be a problem for someone setting something up for the first time without access to the machine to be remotely controlled or the network it's attached to.

When I set my parents' Mac up, I suppose I may have had to open up a port on their router, though I can't remember for sure. The big time saver for me however was setting up the router with a free dynamic DNS service so I could slap an easy to remember sub-domain on it - which obviously stays constant even if their IP changes.