She was "stronger" in that she was more confident in herself.Yes it was, but Buffy died, was revived and got stronger? I hope the next episode explains that one.
Other than that, yeah Prophecy Girl was a fantastic episode. It did what a finale is supposed to do, culminate the season and have a really cool climax. I have to say I wasn't entirely a fan of Xander though. Since when was he interested in Buffy. I thought it was Willow.
The main theme throughout the first season was her trying to find accord between the slayer part of her self and the normal high school student part of her self. It was her initial rejection of Giles. Her wanting to return to cheerleading. Her inability to keep up with her biology homework, even with her teacher rooting for her. Her trying to date Owen. So on and so forth.
In the end, it was the slayer part that killed her, but it was the "normal" boy (and not Angel) who breathed new life into her. The two sides of her working together.
"I may be dead, but I'm still pretty." Was her saying (to herself) "I may be a mystical badass from a long line of mystical badasses, but I can still go to prom."