I really couldn't imagine BtVS without having a season 6. It's that season that showcases the sides of the characters that we had yet to see - the dark sides.
Xander and Anya think they're all ready to be good and married but when it comes down to it, Xander is just a "scared little boy" and Anya easily finds her way back into the demon side of her.
Willow goes all drug addict with the magic and has the worst relapse I could ever imagine - and that's continuing the "what if" from season 5 since Tara had already brought it to Willow's attention that her magic was starting to get out of hand.
If Buffy had stayed dead when the series ended, I personally would have been none too happy about it. I mean, I really didn't like the little game her and Spike kept playing. Besides that, this season tested her relationship with her friends and Dawn in relation to what she was feeling but wasn't showcasing. That was interesting to see how everything unraveled. And I still find myself tearing up when she tells Dawn that she just wants to show her the world.
And Giles tries to leave how many times? I didn't really like that, seeing as to how close the Scoobies had grown. Sure, Buffy wasn't doing the things she was supposed to, however, that's when he should have put his foot down no matter what. She needed to learn responsibility if she was to survive in the real world.
Anyway, that's my two cents.