I've run a server for me and 3 others for over 7 hours now. Works almost perfectly, except a millisecond of lag that you really notice with the Demon Eyes and Slimes. Other than that it's running good though. I too port forwarded but no one could connect, so I had to set my router to DMZ. I hope they get that sorted out.
Intel i5 quad
4 gigs of ram
Decent cable internet
Loving the game so far. This is the kind of game you need good friends to start playing with though. Having someone come into a new game with basic gear like grapple hooks and mining helmets can annoy you pretty fast, especially when they're only focused on getting what they can for themselves. They zip ahead of you and gobble stuff up and you struggle to keep pace, and then are like "oh, you're not running 2x as fast, have an ambient light source and can grapple tall mountains? Oh, right. Sorry."
Then they want to go start breaking shadow orbs while you're still trying to make a metal weapon, or build your first house. This game is great if people work together. It's kind of cagey when they don't. When people have maxed out characters, I'm not sure if it gets better or if that's when the griefing begins, because since everyone has their own little world to go back to, there's not a ton of reasons to play cooperatively unless people really care about doing so.
Bottomline: play this game with people who like co-op, not people who just like to have someone to talk to while they play.
And the spawn numbers...my god, they're horrendous. 4 different people with 5 to 6 spawns on them a piece. There is definitely some spawn scaling going on relative to player numbers.