The part I love most about it is that you can be such a glutinous messy over stuffing fajita freak when you place all the ingredients out on the table.
There are endless ingredients and salsas etc that you can make for fajitas. I always tend to go with the marinated chicken my mother used to make us but you could replace that for strips of beef, pork, fish (my personal favorite when I eat real fajitas in the States).
The crucial parts to this dish is that you have tortillas (if you can get hold of corn then you are lucky - use 'em) guacamole, sour cream, beans or rice and some sort of protein - preferably marinated in some sort of mexican related spice mix.