The show started off strong. It was, in a way, psychedelic. Among that, people who have been following the show since the beginning got to see some shocking things as the story grew.
As the story grew, the lore did not catch up. The writers then, i suppose, kind of bullshitted a few formulaic episodes.
You know how the critics say a "normal" AT episode goes–first it starts off with Finn and Jake hanging out saying adult-friendly terms like "bro", then something pops up for no apparent reason that needs help. Then without too much action they resolve the conflict, and/or its a romance episode. I personally believe this to be partially true.
However, I have grown to love the Ice King episodes. He's just this awkward old man who wants a friend now, and he's batshit insane. But chronologically, it seems his former self shows through more and more. That's why they added Marcy. Those two episodes were probably the best in Adventure Time. And I've liked AT since it was still a wedgie on CN hosted by Frederator on youtube.