When adults describe a what is nominally a children's show as multi-layered, what we usually really mean is that it has all of two layers: one for the kids and one for us. Adventure Time is so much more. Don't be fooled by the extreme minimalism of the artwork or the simplicity of the basic storylines: this is a _very_ complex and thoughtful piece of television. Not every single episode lives up to the standard I've just described. But as a totality, the season most definitely does.
Adventure Time is the kind of show you can watch as an adult, see maybe half a dozen episodes and think you know exactly where you are. Then along will come an episode that blows away your entire understanding of what it is you're watching. Then, maybe another half dozen episodes down the track, just when you've settled down and think you've regained your bearings, along will come yet another episode that once again changes everything.
And so on.
Perhaps fittingly for a show that will turn many a grown-up's understanding of what a cartoon can be on its head, Adventure Time is entirely uninhibited in referencing an incredibly broad range of cultural touchstones. Whether making subtle "in" jokes about Dungeons & Dragons or drawing on Aristotelian theories of tragedy, if there is one thing that Adventure Time is not, it is a show that assumes that the viewer is stupid. The thing I personally love most about it is the supremely gentle way that it lets us know that yes, there is real darkness in this world, even as it encourages us to confront that darkness with courage and personal integrity; and without losing our own sense of joy and playfulness in the process.
In the end, we might almost say that Adventure Time is a show with as many layers as the viewer him or her self is ready for.
If you miss out on this one, you'll really have missed out on something special.