The way the characters in Adventure Time were handled was loving terrible. The show had some of the better characters of any of the shows on Cartoon Network during its run, and it seems as though just from looking at the finale song (I stopped watching after Flame Princess got her kingdom almost taken down by Keith David because it was clear they weren't going to develop her any further) that all of their storylines end pretty disappointingly.
Finn gets bounced between fifty-billion different girlfriends and is never able to settle on one because of stupid plot bullstuff. I'm pretty sure that Flame Princess would've understood if Finn just told her right off the bat that whenever she and the Ice King fought, the Cosmic loving Owl came to him in his dreams, but he never tells her until it's too late because it would make too much sense to tell her right of the bat that an omniscient cosmic entity was visiting him and to understand why she'd need to attack the Ice King again. I checked in like several seasons later and he's interested in this huntress wizard person or whatever and this is the perfect time to allow him to settle down and finally be in a long term relationship but looks like that doesn't happen because Pendleton Ward is a sadist when it comes to Finn for whatever reason.
The Lich is treated like Sauron or Voldemort for the first couple of seasons. He's this all-powerful demon that can't be killed by conventional means and whenever he comes back he only gets temporarily defeated by Finn due to either pure luck or pure adrenaline. Then he gets turned into a giant horned baby and you think, "okay well at some point he's gonna come back, he's the one who destroyed the world after all", but it looks like he never does. That's like if Darth Vader and Luke didn't fight at all during Return of the Jedi.
PBG and Marceline making out sounds like fanfic levels of writing.
I think the problem is that they kept trying to introduce more and more new stuff and by the time the series had ended they had forgotten entirely about certain integral plot points that they set up. What a letdown.
