Great question. This actually brings up a few points that I hadn't really considered much until now.
Buffy spends the entire episode criticizing everyone in the house for not "doing more;" for being weak and for letting evil overpower and intimidate them. It is profoundly moving, and a frank indication of exactly how serious she is, when she berates Spike for not being aggressive enough--for not being the vampire she met all those years ago. She wants him to allow himself, essentially, to be taken over yet again by the demon inside, in spite of all of the work he'd done to abolish it. I always found the exchange between them to be heartbreaking; here is the man who has found a way to be good, and the very woman who inspired him to do it is now asking him to revert to being like the thing that was her enemy.
Then comes the point at which the tables are turned. Buffy has an opportunity to practice what she preaches. To take more power without regard to what it will do to her as a person, or to the implications therein. She knows what she's up against, and she's just asked her friends to make any and all sacrifices necessary to prepare for war. But when it comes time to, (as [MENTION=11292]kittenpoker[/MENTION] pointed out), essentially allow herself to be raped by these spirits--by the power--she refuses. It's too much. In spite of the cost, she won't let herself become a monster. As a slayer she now knows that her physical strength is already rooted in darkness, and she is unwilling to discover what increasing that side of her heritage will do to her. To the woman she's become.
Buffy is all but horrified to learn how much of her power lies in darkness. Yet come "Chosen," her solution is to imbue all potential slayers of the world with this very thing.
The hypocrisy in all this is very moving-- as it applies both to the end of the series and toward her exchange with Spike--and I think it needs to be acknowledged.
Ultimately though, I am glad that she refused the power. It was the honorable and stronger thing to do, if not necessarily for the greater good. It's one of the many reasons why we love Buffy: one of her greatest strengths is always her belief in herself.