The third installment of Buffy started with the re-introduction of it's heroine. It establishes for the remaining five years that Buffy really isn't suited to this. She's not the best ever Slayer, she's not the most focused, the most driven or motivated, she's just one in the Slayer line. And to make this point? Cue Faith.
Shorn of family or social ties, Faith should be far better suited towards Slayerhood than Buffy, viewing it from a self-perspective that lacks the weakness of dependancy. When Giles is sacked from the Watchers Council, you understand the Council's fear of people getting too close to a Slayer.
Love features heavily this season, with Willow and Xander's attraction coming to a boil and subsequently ruining their relationships, and Buffy acknowledges to Angel that she can do nothing but love him, prompting his departure in the final episode.