I don't know if I could ever quantify an answer that would be anywhere close to what causes me to say good mechanic or bad mechanic.
Variables include but are not limited to what type work I'm wanting done on my bike, how thick my wallet is at that time, previous experience with that mechanic or shop. How well they talk, which might just include interesting stories and other conversation even if the mechanic skills are slightly lacking. Maybe even which side of the bed I got out of and the color of the sky that day.