I've seen this show mentioned on the forums through the "Image Thread" and other random posts, but it hasn't gained its official post yet, so here it is.
After watching the Christmas Special, "Holly Jolly Secrets", I can't help but notice H.P. Lovecraft's influence on the show. There are other obvious influences, like Dungeons and Dragons and post-apocalyptic settings, but Pendleton Ward ended this episode with a Lovecraftian flourish. I don't want to give any big spoilers for the episode, so just watch it.
There are other Lovecraft nods in the show in different episodes to boot! In "Mortal Recoil", the Ice King mentions his "stanky old wizard eyes", and then gives his point of view on the matter:
http://adventuretime.wikia.com/wiki/Wizard_Eyes. This links with Lovecraft's short story, "From Beyond", where a mad scientist creates a machine that allows the user to see into planes of existence beyond reality. The protagonist in the story sees an alien plane transposed with ours, where indescribable creatures flitter and shamble about. The catch being, if you can see them, they can see you.
The father of Marceline, a female half-demon half-vampire, is nothing but a soul sucking horror from beyond who is summoned through an intricate ritual and banished in the same manner.
There are more examples, but these are reasons why I love watching this show.
