"A good oil & gas well" is any well which is drilled into minerals you own.
Now with that being said, it really does matter where you're located as to what might be considered "good". In many places a well with an I.P. (initial production) of 300 bbls of oil per day is "good". Yet in the midst of the Bakken play many would consider that same well a disapointment.
So the level of expectation, and where you're located, matters as to how a well might be judged. There is not really a hard and fast standard nationwide. Personally I'd say an I.P. <75 bbls is "poor", an I.P. >400 bbls is "good", and an I.P. >2,000 bbls is a "monster". No doubt others would have entirely different ideas on this.