I also played multiplayer with ThorRune (Zetor2003) but I noticed some difficulties, which I have suggestions for:
1: The client should not specify what port to use in their config. Only the server should, then the client simply in-game should be able to say: connect to ip:port or hostname:port (internet and lan).
2: You mentioned NAT, this can be solved these days by adding UPnP where a port is dynamicly allocated and used to host the game.
3: Notification on the server, when someone joins the game. It was annoying constantly checking the player list to see if the other person connected or not.
4: In-Game messages, essential in any game

5: Basic management server-side (Kicking players, banning players and IP's, passworded games, etc).
But besides those, great game you've made there! Nice work.