My Buffy Theory
That Buffy came back alive without another Slayer to take her place. A lot of people say it was poor writing or a mistake. I think the last season rocked, which is also why Angel suffered because the good writers can't all be in one place. Sometimes I pretend that season four of Angel didn't happen, it's a happier place, a place where things make sense and your favorite characters aren't engaging in statutory rape. But I'm out of that place now and back on topic.
I think another slayer didn't take Buffy's place when she died the second time because a) it was a mystical death and b) she had already died before so therefore she didn't register in the slayer succession anymore because Faith, yeah remember her, she was still alive. Now I'm thinking if Faith dies then another slayer would take her place, but this no longer matter thanks to one of the best tv series finales ever.