I like it because it's fun and also both the girls and boys get to have interesting personalities. I don't know if I'd call it all that deep and meaningful, but it is nice that the show has something more to offer than just random humor.
Allegory is not truly present in all works of fiction, many of those works that appear to have deep allegorical symbols and messages are not necessarily intended or focused on. But in the case of Adventure Time I'm pretty sure it is more of a creative approach than an Allegorical one, so they might slip in a deep concept in an episode or two.
I liked that episode where they go into someplace like hell, where a lazy guardian guards the entrance. Also love all the original and creative concepts. Very inspiring I say.
No, really Some of the messages are more of just "the moral to the story" type things. I only just recently got into the show, I thought it was stupid and weird before. Not saying you have to like it, just saying that this person isn't making stuff up.