3. BMO’s gender fluidity was no big deal
Many characters in Adventure Time are playful with gender. In Season 5 Episode 18 Jake experiments with makeup, and who could forget the first time Finn showed off his hair in Season 2 Episode 10? But no character embodies gender fluidity better than BMO, the loveable sentient videogame console who's constantly playing make believe and providing the show with a dose of childlike joy. Referred to as "he," "she," "m'lady" or "young man" across varying episodes, BMO's playfulness with identity is so incredibly earnest and pure that it's guaranteed to hit you right in the heart-guts.
A whole bunch has been written on BMOs' gender (including an academic paper!). The consensus seems to be that because the characters in the show don't treat BMO's gender as a big deal, it isn't (and shouldn't be) therefore Adventure Time is able to communicate some important messages about how useful the male/female binary really is in defining ourselves. For children's television, this is pretty special.