Know all about common carriage and holding out. For exmaple, if some guy walks into the FBO and asks you if you can fly him to an airport 100 miles away every Sunday afternoon could you do that? What about if one week he wants to bring his buddy from work? What if the next week he brings two people he charged $25 bucks to ride along? What if his plane breaks, can you use a rental plane from your FBO instead?
Another area that is usually weak is aerodynamics and weight and balance. Will the airplane stall first with a forward CG or aft CG? What is angle of incidience? What is washout? Where does the wing stall first, root or tip? Where is the center of presure. Does the tail give you negative lift or positive lift? What is a stall? What is a spin? What is the difference between a spin and graveyard spirl? What is the one instument on the panel that would tell me which one I am in? Make sure you know the weight shift formula.
Lastly, know your systems. If I pull the landing gear CB, will the gear stay retracted (Piper Arrow)? Why does the MAP rise when the RPM is decreased during run-up? Is it alright to run the engine with the MAP higher then RPM? (yes in most cases, check the POH for power settings). What happens to the prop setting if you lose oil presure. What prop setting creates more drag, low or high? What does MAP read when the engine is not running? (suprising how many people this catches) If you lose electrical power what gauges on the panel will still work?