The basic abilities required to ride dirt bikes are quite obvious:
Balance is the most obvious of the basic skills. You need to be able to balance on the bike and keep it upright or leaned over in a variety of conditions.
Riding motorcycles requires more focus than say walking, that much is obvious. As you progress to offroad riding, the need to stay focus is much higher, due to irregularities, obstacles and other green things trying to take the bike from your control.
You need to be able to maneuve the bike and pick it up when you drop it.
Endurance comes into play more during races, but also in the beginning. Beginners tend to get a lot more tired and fast. You need to put up with it long enough to get the experience...