Besides the fact that the creator(s) are clearly geniuses, I think a big part of the awesomeness of the show is that those creators are abstract people. One example was when Finn (the human) suggested that they should pretend to be defeated by a tiny little army of little mushroom people because he pitied them and wanted to give them confidence. To this, Jake (the dog) said something like "no way man, it'll ruin my stats".

Or NEPTR's "You cannot tell, but I am giving a thumbs up." http://www.youtube.com/watch?v=qTTplK3kin8

In other words, the humor/atmosphere is much geekier than what you'd expect to find on a television show. The only other show I remember with this aspect is Futurama, except that Futurama is overt with its jokes, whereas Adventure Time has many shades of subtlety.