I think I know what you mean. Adventure Time is totally out of order for me, but it seems like it's probably what you're referring to. Everything's just "totally 'natch, bro" vs. when the show first started and was still aimed at its target audience and not adults who "get it" and are "down with it.."

I've been hearing a lot of hype for this series. I must check it out.

The ending is mythic in a way that is hard to describe. A lot of the show is inspired by American 19th century children's literature, which in itself was always this weird mix of extremely disturbing and innocent. The show reflects that. The last two episodes really got to me and maybe it was because a relationship of mine had just ended but very few pieces of animation get to me like this one did. The ending felt like a sucker punch followed up by some tea.

Also in one of the first episodes, Chris Isaak plays a pumpkin demon.