It was the season finale to end all season finales.
The episode left a lasting imprint on pop culture; years later, the famous cliffhanger was still being parodied on series like The Simpsons, which ended its sixth season with “Who Shot Mr Burns?”
But season five remains the most shocking finale of all, as Buffy ended up sacrificing her life to save the world.
There is more scope (and less pressure) for writers when approaching a season finale as opposed to the actual end to a series.