I don't know why you're all saying Buffy's character hasn't changed and developed. I just saw season 1 and that is exactly the thing I noticed, only the other way around. In season one, all Buffy cared about was clothes and boys. She didn't want or take the slaying very seriously. She was so much like a kid, she WAS a kid, and she acted like it. Then I look at season 7, the last season, and surprise surprise, she's not a kid anymore. She became a General, grown up (grown up especially when her mother died and she had to be Dawn's mom), she took slaying more seriously, and that all that mattered to her. She put all the other things aside. Dating wasn't her first priority anymore. She dated, but the world came first.
She went from a brat kid who got exited of a piece of clothing, to a responsable teenage who got kicked out of her home and living alone in the big city, plus killed her first big love so the world could keep on living. To a senior who's looking forward to her college years, to a grown-up (even Joyce said that) that takes her studdies seriously plus the slaying, even when she didn't want to, but understood she has to because that's her destiny, to a mother to Dawn, understading that she has to sacrafice herself to save the world, then when coming back from the death, she had even more responsability, she took a job when she understood she didn't have the choice, her life was a bitch and she didn't want to live (after season 1, Prophecy Girl, where she says she wants to live) Her eyes got open to so many things, and especially life itself. To a General who's heart-blocked by the seriousness of her destiny, she wasn't Buffy that we came to know (and love?)
Wow, there is so much more to say on this subject, but I won't bother you no more. My point however is, Buffy did develop, she did change, and her character is so deep and complicated that there is no room to tell it all. Every episode you watch you can pick up stuff from Buffy that prove she has a whole world inside her, not just a shallow character. I think she's more complitated than anyone else on the show. She is the Slayer, what's so empty about this character?