I remember once it was the rudder pedals that were really too "sluggish" and the instructor grounded the plane, another time I taxied the airplane back after the engine run-up (I really didn't like the sound), and the rest of the time it was inoperative equipment noticed during pre-flight checks (like fuel gages, nav/strobe lights, radio, etc). Not necessarily a big deal you might say, but since on a non-MEL airplane everything is supposed to be working, I'm not playing with my licence and my life.
By the way, when I was there a student and his instructor killed themselves in the nearby mountains during an IMC night approach into La Verne. I'm still waiting for the final report by the NTSB, but most probably they'll say pilot error, visual illusion, and loss of situation awareness or something like that. Sad incident which unfortunately fueled the bad jokes about ADP (supposedly meaning "Another Dead Pilot") because they don't really deserve a bad reputation.