The scene where Sheldon and Leonard observe Penny through her open apartment door would be repeated as an homage in the series' 100th episode. The characters would even be dressed similarly, right down to Penny wearing the exact same blue top that she wears here.
While Penny has a pleasant outgoing personality, Katie from the unaired pilot opening was meaner and mocked them. Penny does have a temper and some of this personality came over from the first pilot when she's really mad.
Sheldon worries that his offspring via the sperm bank might not know whether to use a differential or an integral to determine the area under a curve. The correct procedure is to use an integral. A differential would measure the slope of the curve.
Sheldon uses the term sex when talking to Leonard about Penny. When Howard arrives, he uses the term coitus. In later seasons, the characters would reverse these terms with Sheldon exclusively using coitus.
Raj's hat in this episode has the number 42 on it, likely paying homage to Douglas Adams' book Hitchhikers Guide To The Galaxy where the answer to the ultimate question of life, the universe and everything is 42.