There is a certain school of thought that says that none of what Finn is experiencing is real, or is at the very least vastly distorted from the true (and likely terrible) reality he lives in. Word of God states that Ooo is a post-apocalyptic world recovering from what is referred to as the Mushroom War. The possibility exists that what Finn views as adventure might be a great deal more harrowing than his mind allows him to process... throughout the series, there is all kinds of Scenery Gorn, with destroyed or submerged buildings, half-buried and wrecked cars, broken televisions, abandoned military equipment, and notably, a river of trash. The opening sequence briefly features a barren field of bombs and bare trees, as well as an arm reaching out, and several of the situations in the show could be read as euphemisms for what is really taking place (such as the Rainicorns attempting to eat Finn, then substituting him for 'soy people' as a euphemism for cannibalism; the Ice King's obsession with kidnapping princesses as a euphemism for slavery; the relationship between Finn and Jake being perhaps a master-slave relationship with Jake the master; the absence of Finn's (and most other characters') parents, the fact that he lives in a tree; possible deformity or mutation represented by Candy People and various other non-human characters, etc). Word of God states that this isn't the eventual revelation of the series... while throwing in sounds of Finn screaming and machine-gun-fire over a freeze frame. There are some similarities between Adventure Time and A Boy and His Dog, as well. Is he suffering from radiation-poisoning and starvation-induced hallucinations, and repainting his life as a coping mechanism, or is it all true with the context of the story? Who knows.
Princess Bubblegum may well be the most divisive character in the whole series.
A prominent section of fans follow the school of thought that Bubblegum is a Bitch in Sheep's Clothing. For one thing, she treats her creation/"son" Lemongrab as an insane freak, and though she is more or less right, her seeming lack of compassion towards him and apparent unwillingness to seriously help him makes her come off as negligent, if not outright abusive. (For what it's worth, Bubblegum is shown to regret how her mishandling of Lemongrab's situation made things worse, and did try to make it up to him, though that opened another can of worms; see below). Her handling of Finn is also questionable, seeing as how she rather callously told him to "move on" during Too Young and to "stop acting weird" in Incendium.
Or, Princess Bubblegum is simply a verystressed out young woman who has a lot of responsibility on her plate (being the sole ruler of a whole kingdom and trying to keep her subjects safe from all the weird calamities that occur on a frequent basis, for starters) and no time for a personal life. It's also clear she really fears the possibility of dying and leaving her kingdom under the rule of an Inadequate Inheritor, which may go a long way in explaining her disappointment with Lemongrab. It's telling that Bubblegum's own voice actress has described her as "selflessly evil" - she means well, but isn't afraid to do bad things to achieve a good end. As for Finn, she could just be clueless about his feelings and/or refuses to date him because he's still younger than her, and is just trying to let him down with as much tact as possible. That, or she's a lesbianwho broke up with Marceline.
This actually happens In-Universe as well. In the episode James, PB basically convinces the title character to sacrifice himself to save herself, Finn and Jake, and then commemorates James by cloning him. Was it as justified as she claims, or was she going over the line regardless of her good intentions? Finn and Jake themselves aren't even sure.
As for Marceline herself, it's well known by this point that she survived the Great Mushroom War and was just a very small child at the time to boot. Marceline could be considered the only sane person in all of Ooo, as her main motivation seems to be just hiding from the world, and not getting involved, as an apocalypse survivor's primary motivation is generally self preservation.
What is Princess Bubblegum's relationship with Marceline? "What Was Missing?" heavily implied they had less than platonic feelings for each other, and future seasons further imply it. Still, there are many fans who think they were simply best friends who grew apart over time. If they are in love, that still leads to many interpretations. One (supported by Marceline's VA) was that they were a couple once but broke up prior to the series. Another popular interpretation is that they were best friends who slowly fell in love but broke off their friendship before they could date, leading to a lot of Unresolved Sexual Tension. At the end of the series, their past relationship is still unclarified but they do undergo a Relationship Upgrade.
It is confirmed they have dated in the past and rekindle their romantic relationship in the finale of the series.
There's also the question if Jake's laziness is actually due to him being afraid of being so powerful that he would constantly receive instant-gratification in everything he attempts, and thus would end up completely jaded with life (as he insinuated to BMO after he got a shard of tortilla chip stuck between his teeth), or if that is just an excuse and Jake just genuinely enjoys being lazy and putting in as little effort as possible, wasting his incredible talents.
Ice King: A loser and jerkass who always tries to force princesses to marry him, a tragic Woobie who just needs a hug and a reason to be happy, or is his mind just too muddled by the crown for him to fully comprehend that what he's doing to other people is wrong?
Ice King: I don't know... I'm complicated.
Lemongrab:
Safe to say he's quite a bit smarter than he would let on, but to what extent? Word of God confirmed that Lemongrab was the one who built the Sound Sword. It's not far of a stretch to imagine Lemongrab could even be a Genius Ditz, as it has been demonstrated in Real Life that autistic people can be incredibly skilled in some specialized areas.
In "Too Old", did Lemongrab abuse his children and his clone because of his extreme mental problems? Or was he lashing out because of his grief over Lemonjon's death? Considering the thoughts he had in his brain after Lemonjon's death ("RAGE," "SUFFER," "call me crazy, but it's time we die") supports this.
Related to Lemongrab, is Princess Bubblegum keeping a hands-off approach to him due to the complexity of legal treaties, or because she simply doesn't want to have to deal with him? Most of the time Bubblegum can't stand to be in Lemongrab's presence for more than a few minutes, and leaves him to his own demented devices as soon as a convenient band-aid can be slapped on whatever problem he's causing: this ranges from dumping him in another castle after his botched creation, to creating a second just-as-flawed Lemongrab (which made the problem far worse), to barely noticing that Lemongrab was electro-shocking his citizens and had half-eaten his brother in her rush to get away from him, and finally to sewing him back together and then abandoning him yet again.
Before her first physical appearance in the series, Betty got quite a bit of this, due to her having a pretty big role in the Ice King's backstory. The interpretation boiled down to whether she really was as nice as Simon made her out to be, since she left Simon after he went on some kind of nutty rant under the crown's influence. That interpretation was completely Jossed in her own title episode when it was revealed that Betty never truly left Simon, simply jumped into the portal to the future to be with him, and showed her dedication to Simon by promising to find a way to free him from the Ice Crown.
Her potential relationship with the Ice King now that she has Magic Man's powers that we now know is linked to "madness" and "sadness". Is it better for her to become as crazy as the Ice King and let them be crazy together forever and Simon never being healed or is that far more tragic than him just dying? Is the only really happy ending her healing him? It really all comes down to how the viewer interprets the effects of M+M+S.
To some, Braco was a massive woobie who was absolutely deserving of PB's love, and was unfairly rejected. To others, he's an entitledstalker, and was way too willing to settle down with the PB-Bot (casting doubt on whether he really cared about her as a person in the first place).
Was Flame Princess right in dumping Finn after he "betrayed" her, or did she grossly overreact and let one bad experience automatically cancel out all the good times she and Finn had together?
Additionally, is she really over Finn? Or is she lying to herself, and desperately trying not to be a Hypocrite by breaking her own "total honesty" law?
Vampire King: was he being sincere in that he wanted to break the cycle and remove vampirism from the world, or was it all just a ploy in order to release his pure black essence of vampirism onto the world? Alternatively, maybe both are true; perhaps Vampire King was already aware no matter what the dark cloud of his essence would always be a plague onto the world, but wanted to remove himself physically from the equation. The wheel of fortune still exists as it always has, but he removed himself from that cycle.
