A martial arts school that is run for the purposes of making money without teaching true martial arts. The most common signs of a McDojo include a lack of sparring classes, no opportunities to compete in a tournament, outrageous fees, and a promise on having a black belt in 2-3 years. Earning the black belt in 2-3 years, depending on skill, is quite alright in many systems, as there are always highly talented pupils. However, the promise from first glance is a sure-sign.
Not all Taekwondo schools are McDojos, despite popular belief. Pre-School programs, American/Korean flags, mirrors, and the lack of an Asian instructor do not equal a McDojo. I study at a school that has all of these, and nobody would even dream of accusing my school of being a McDojo. The ignorant martial artists that like to blam Taekwondo because it is a newer style and because it's popular have no clue. There are Kung Fu schools that fingers should be pointed at. There's a perfect example in DengFeng, the Chinese Martial Arts City. The monks will sign any foreigner off as a master just so they can get a good laugh. Taekwondo is not a bullshido style. We taekwondo types are just scapegoats for the ignorant martial artists of other styles.
John's sensei left his teacher when he was only a blue belt in Kokyuroshin Karate because he thought he was good enough, even though his own personal skills lacked and thus caused John to get killed while trying to defend himself. John was the product of a poorly led McDojo.