The article emphasizes the importance of subject lines in email marketing, citing the Pareto Principle that a small number of factors can have a disproportionate impact on success. It argues that startups often underestimate the power of subject lines and instead rely on generic, impersonal emails that fail to grab attention. The ideal subject line is short, clear, and direct, with studies showing that emails with 5 words or less have higher reply rates. Personalization, particularly using the recipient's first name, can also increase engagement rates. Effective subject lines ask questions, spark curiosity, and clearly describe the content of the email. Ultimately, the article advises against relying on gimmicks or growth-hacking tricks and instead recommends following a simple golden rule: describing the subject of your email in the subject line itself.