The f7 / f2 Weakness

Early on, the f7 pawn (f2 for White) is defended only by the king, so it's the natural target of a quick attack — a bishop on c4 and a knight on g5 both eye it. Letting a piece bear down on f7 with nothing but the king to guard it invites sacrifices and forks. Develop and castle before it becomes a hole.

Source for further study: classic opening lore (Italian / fried-liver motifs).