From both an engineering point of view and piloting, the Hyd system is simple and robust, and from most of the reports I've managed to locate, a high proportion of the hyd related incidents are training related. I think the biggest "gotcha" has to be lack of knowledge as to "why" we drill the way we do, and the need to consider the yaw compensator. I know from personal experience how easy it is to get behind the aircraft and be tempted to flick the hyds back in, but quickly reminded that this is a bad idea!