Everyone whose arc hadn't satisfyingly wrapped up in "Islands" and "Elements" got their due, even -- to my delight -- BMO. There was a lot left to the imagination, but since a major point of contention has been the idea that too much of Ooo was being explored and the show was losing its sense of mystery, I'm glad for that (even if I've always thought those criticisms were unfounded).
I'm glad Simon got rescued for good from being Ice King. It diminishes the dementia metaphor a little to give him a stable ending, but the man's been through enough, and Betty's sacrifice was enough of a cost that it felt earned.
And I never expected to see Marceline and Bubblegum get a full-on kiss and confirmation as an official couple. Ever since the "What Was Missing?" debacle, Olivia Olson's outside confirmation, and the reasoning that the show airs in places where homosexuality is illegal, I'd resigned the duo to just getting endless subtext teases while other cartoons pushed the boundaries. They certainly waited until the last minute, but I couldn't be happier with it.
I teared up a little over Rebecca Sugar's final song, "Time Adventure", and it was effective enough that it overpowered my brief annoyance at the trope of the evil force being countered (however briefly in this case) by everyone singing. And then the show did exactly what I expected it would, setting the ending montage to "Come Along With Me" -- the credits song I've heard hundreds of times -- and it completely broke me.
Ultimately, there was no second apocalyptic war, despite what was foreshadowed since the show's halfway point. Everyone just kept living, and the old and overgrown Ooo we've occasionally glimpsed was just that: old age, a time long past the era of Finn and company. On the one hand, the idea of history repeating was beautifully terrifying, and lent a serious, ominous edge to the later seasons. On the other, everyone being able to dodge the mistakes of humanity and live on with the lives they've built is a great thematic endpoint -- if not as brutally daring and subversive as having them all destroy themselves would have been -- and I'm not sure I would have been able to handle the sight of Finn melting in the shockwave of a mushroom cloud. The treehouse being destroyed and Fern's last seed being planted in its remains filled my devastation quota fine.
It's not a perfect finale by any means. I'm annoyed that the Lich was easily dealt with right before the final season, despite so much buildup; Uncle Gumbald was a lame antagonist who was introduced way too late in the show to be built up, Fern was great but didn't get enough to do as a villain, and while GOLB was intense and I'm glad there was payoff to years of ominous glimpses, his silence meant he couldn't hold a candle to his disciple. The dream sequence / mental battle with Gumbald and Fern dragged on too long, giving fuel to the detractors who say the show is pretentious. And I wish such a colorful, imaginative show had had its final battle with an eldritch abomination happen somewhere other than a fairly bland crater. But those flaws are ultimately overshadowed when there's so much good.