One thing that I've always liked about Adventure Time is that it managed to be surreal in an upbeat, friendly, and funny in the kind of way way that made it fun to watch because you weren't supposed to take it seriously, and you couldn't if you tried. But that was in the beginning; now the show has just gotten a little too plot heavy. It still has the surreal whimsy, but it's just become less funny and fun. Although, I'm not saying I don't like the new plots: the writing is surprisingly incredible and I love the hidden depth of it all, I just wish there was more humor involved and occasionally the lighter plot episode like there used to be.
And I don't like how serious Finn is now, he's become too angsty and mature. I guess they had to do that since Real Life Writes The Plot and his actor sounds much older now, but still.