1: Pretty much anything, stories, art, videos if that's your thing (links anyway, not embedded). It doesn't even
have to be from the X universe, although I'm not sure what kind of reception you'd get for anything else
2: DiDs are the single most exciting way to play the X games. Because it's
SO easy to die if you take your eye off the ball a lot of folks like to record their exploits, some write stories around them, others just like a straight action report style. Because we're all X gamers we can all appreciate somebody elses experience
3: Because there are so many different ways of playing, especially with the use of mods and scripts, we generally list rules etc so readers don't get confused. I think Dragondream said in his original DiD listing 'If you're playing with the Battlestar Galactica mod and haven't made your readers aware of it they'll be a bit confused when you announce the purchase of the Galactica'. Also, because a lot of us have played the game to death and know most of the exploits we can avoid things like this: Day 1 - Mined Nividium, Day 2 - Mined Nividium, Day 3 - Bought myself a sick ass fleet.

The whole 'Pilots Wanted' thing...I'm not sure where that started, but making a story at least partially interactive makes it more fun, and adds another element for the author to play with. Check out Scion Drakhar's 'From Nothing' for a perfect example
4:

No.

Anyone is welcome to post here.