Flip flops can be dirty, non-supportive, and allow the foot to function in very inappropriate and damaging ways.  Non-supportive sandals and flip flops tend to allow the foot to excessively pronate, creating strain and even stretching and tearing of certain tendons, can contribute to bunions and hammertoes through muscle imbalance, and will most certainly lead to pain and injury in people who walk or exercise too much in them.  Pronation is very damaging, and consists of 3 motions of the foot.  Pronation consists of Dorsiflexion of the ankle, Abduction of the forefoot, and eversion of the subtalar joint and heel.  So when you look at a pronated, or flat foot from behind, you see the heel everted (pointing away from the midline of the leg toward the arm or outside of the body) and the Achilles tendon will have a medial bow instead of being linear.  You will see (still looking from behind) the ‘too many toes’ sign, which is abduction of the forefoot, and the arch will at least flatten to a certain degree, if not completely be flat and there will be a medial bulge of the talar head toward the floor.  I will write more about the long term affects of pronation at a later date, but link into this article to read a little about how bad flip flops can be fore you.