I won't do a full list of everything that could be covered as it could be endless, and some of the lists already mentioned are quite comprehensive. Rather, below are a few things that seem to have many people (including myself on occasion) either scratching their heads in confusion, or banging their heads against a wall out of frustration.
-How to adjust control gains to have smoother, more professional panning and gimbal control.
-When to replace props; there have been a number of members who have had minor incidental prop contact that left visible marks, but didn't know whether or not it warranted replacement.
-How to assess and understand diagnostic information from the flight records; I only recently started uploading log data to healthydrones.com, but quickly found out the vast information at one's disposal that sits inside the log files to assess the health of the battery (e.g., major vs. minor cell voltage deviations), signal quality, GPS satellite count, etc.
-Limitations of our phantoms, both physically (e.g., flight ceiling, fighting winds, RTH speeds vs. battery consumption) and legally (e.g., FAA regulations, the jurisdiction of local law enforcement/officials, and the legal requirements to use them privately and/or commercially)
-Traits of the LiPo battery, how it's affected by temperature, self-discharging, and how to maintain them between flights
-Which environmental factors impact range (e.g., local wifi, humidity, etc.) for the different signal types (e.g., wifi, LightBridge, etc.), and how to mitigate/reduce the impact on operation
-How to access this site for information, and how to use the search function!