I do think that some of that laughter at the end was joy along with the obvious defiance.
I rewatched both "End of Days" and "Chosen" back to back, and it was interesting how much the idea of aloneness vs. togetherness in EoD jumped out of me. Faith's comment about being alone. Spike's "were you there with me" and Buffy's "yes."
Another thing that jumped out at me at the end of "Chosen" was one of Willow's final comments. [paraphrase] "We didn't just save the world, we changed the world." The Slayer no longer has to fight alone. Women no longer have to play by/be victimized by the old rules.
The more I think about it, the more the idea of togetherness/sharing of strength crops up throughout the seasons of Buffy.
It is mentioned several times throughout the seasons that Buffy is unique among Slayers in that she has a group of friends fighting alongside her. Off the top of my head, I can recall Spike's comments in S2 about the friends thing being a bit of a shock. Then there was his whole analysis of the situation in "Fool for Love." Buffy has friends who give her a reason to keep fighting and who give her the strength to keep fighting. The First Slayer in "Restless" was quite put out about Buffy having friends and sharing her power.
Season by season, the together/alone thing plays out in the final arcs in different ways.
S1: Buffy should have died, end of story, but Xander's being there with the CPR saved her life. The simple lesson is that your chances of survival are better if you've got someone there to cover your back.
S2: At the time, Angel's speech about "what do you have left?" and Buffy's reply of "Me." seemed hella empowering, and in a way, it was. It's also very sad. When her back was to the wall, she was able to fight back, but the end of S2 was a hollow victory in many ways. It ended with an emotionally devastated Buffy leaving Sunnydale with no word to her friends about her fate. She saved the world, yes, but it hardly seemed worth it.
S3: Buffy was acknowledged as class protector, in what was probably one of the very few times a Slayer was acknowledged for the work she had done. Also, rather than the Slayer and a chosen few going up against the big bad, the entire Class of 1999 was in on defeating the big evil.
S4: The biggest threat to the group was when they nearly tore themselves apart from within, and they found their greatest strength when they did the joining spell, using each of their unique gifts to make something greater than the sum of the parts.
S5: Spike changed greatly and let himself be tortured by Glory out of love for Buffy. Willow's love for Tara pushed her into taking on greater--and more dangerous--magical power. Dawn tried to resurrect Joyce even though she knew the consequences might be terrible. Buffy's love for Dawn led her to making the ultimate sacrifice. This season took the whole "love makes you do the wacky" and pushed it to its limits.
S6: Buffy's whole emotional arc was based on her emotional deadness, and climaxed in the grave with Dawn, where she finally felt the need to be part of Dawn's life and to be a sister to her, not just a zombified caretaker. Willow went dark and destructive because of the grief and deadness she felt over losing Tara. In a way, Buffy and Willow shared much the same arc in S6--Buffy's feeling of loss and sadness (one of the things she mentioned about heaven was how loved she felt) played out in indulging in a self-destructive sexual relationship with Spike. Willow's feelings of loss manifested more as outward destruction. Both were brought back to reality by the love of someone who was a close family member. (Xander counts as family as far as Willow is concerned, IMHO).
S7: The hard part of being the Slayer is revealed as not being the whole responsibility schtick, but as the fact that it is a burden that's borne alone. The power of the Slayer was not diluted by being shared--it seemed to be radically increased. A lot of those girls were pulling moves that would have put S1 Buffy to shame, IMHO.
One other thing that struck me: Willow's hair went white as she shared out the power vs. the blackness that manifested as she sucked power out of the books and out of Rack. It seemed to me that she felt pleasure while using the dark magic, but it was a twisted sort of pleasure. The white-haired magic at the end also seemed to be pleasurable, but it read to me as being more of a giddy sort of joy and elation.
Sorry if any of this seems rambly, obvious, or facile. I'm doing my best to avoid packing or working on the stuff I brought home from work.