I've started watching Adventure Time since Season Two, I believe, and I began to watch the previous episodes from then on. In my opinion, the show is hilarious, especially the ones from the first and second seasons. I still continue to watch the new episodes whenever I get the chance to. I must admit, unfortunately, that the show seemed to lose some of the humor it had in the earlier seasons that made me love the show.
Anyway, some other aspects that made me like the show are minor characters like Lumpy Space Princess, and Cinnamon Bun, the fantasy-like air that it portrays, and the morality Finn displays throughout the series. This show is my favorite of the newer cartoons they have today.