The Bare Necessities
The essential requirements for successful aerial photography are a suitable model and camera, with the one mounted securely in the other in such a way as to minimize any vibration from the engine(s) and slipstream. Add a simple, reliable method of setting up the camera, aiming it, firing the shutter and collecting the shots and that's about it.
In reality, although nothing is particularly difficult, there can be a few challenges to overcome before the results are good enough to satisfy the discerning seeker of new horizons.
Probably the most notorious problem is less-than-sharp results due to a mix of camera shake and motion-blurring.