Personally I like the background art and the crazy adventures that Finn and Jake have in each episode. One of the things that call my attention the most is Ooo, which is a very magic place, in all senses, who wouldn't want to live there?
I really like Tittlecards too. I don't know, I just think they're really coll, also I really like the songs they play on them
Another thing that I like about the show is the characters, even the secondary characters are cool and fun.
And I'll be honest, I prefer the older episodes. Of course I don't think that the new episodes are bad, quite the opposite, they are great! It's just that in the old episodes everything was purer and simple, you know? They remind me of my childhood, as I grew up with this show.