So far the game looks and sounds fantastic, but I have no clue what I'm doing. I have two main questions. First, what leader traits tend to be most valuable? I ask this because in Civ3 I know there were several traits that were completely worthless. Second, in the world editor(specifically when editting the Earth map), how do I add more players? I want to make a 12 Civ battle royale for Earth(with me starting in Australia).

Economic is never a bad trait to have. You can never have too much gold.
I'm also a fan of spiritual because I *hate* anarchy.
But mostly it depends on what kind of victory you're going for. If you're going for a cultural victory, then pick a leader that gives bonuses to culture... etc. The traits are pretty balanced in Civ4 in my experience.

Actually if you're going for a cultural victory the best trait you can have is Industrious, +50% wonder production. Gotta have a pile of wonders if you want a cultural victory.

If you want to be military minded go someone with Aggressive. In Civ IV it possible to get any victory with any leader, but if you set yourself a goal and then pick one with appropriate attributes it just makes it easier.

Play through the game on an easy setting a few times and just go for a space race victory, it's by far the easiest to get. You can experiment and change your mind as much as you want and still walk away with a space race victory.

Also, if you're not sure what something does, the in-game Civlopedia is surprisingly useful.