From this episode on, watch John C. McGinley's right wrist. This is the first episode that he wears his "Down's Syndrome Awareness" bracelet. All of season five and on, he is wearing it. McGinley has a son with Down's Syndrome.
The joke about a "horse hospital for humans" in which the doctors walk around the wards with shotguns, is taken from a well known "Far Side" cartoon by Gary Larson. This is an acknowledged homage because J.D. calls the patient he's about to shoot "Mr. Larson".
First appearance of Keith though his face isn't shown and would not make a full physical appearance until the next episode My Rite of Passage.