Quote: "Besides the firewall support, what would you say is the most significant difference between the two?"
Well apart from the two major differences that Multisync is strictly Client/Server, and uses TCP, I'd say it's also much more stable than Tempest.
Quote: "EDIT: how in the world does it add the app to the windows firewall without making anything pop up?"
There's a Windows Firewall API that allows programs to do things such as adding rules to the firewall. However, it's only possible to do these things when using an administrator username.
Quote: "Maybe it's just the way the example is coded?"
Indeed, it is.
Quote: "Not over here, as it appears that the server is not hosted at all when using the external IP. while all the ports are open and ported to my PC."
You can't connect to yourself using your external IP, you have to specify your computer's LAN address.