Power stars give off radiation ( or magic ) and can temporarily increase the users mental and physical ability. Eventually the magic wears off, or is transferred to something else. Bowser used star power to seal the doors, but when Mario got more star power than the doors seals, it cancelled out Bowser's spell. Having lots of star power over a long period of time starts to increase your bad side as well. Since Mario only had the stars for the little time in M64 nothing bad happened to him. Though the resulting effects of 120 power stars gave him probably permanent increased agility, but not enough to make him punch through concrete or jump 10 yards.