These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Broken Base: Over Loki, over whether he's a murderous Jerkass to be avoided or a misunderstood and underappreciated guy who does what has to be done for the greater good, and how big of a role (if any) he actually played in the death of Baldur (which only happens in one of the three different tellings of the story and one of the two that probably is heavily influenced by a christian worldview).
Designated Villain: Being a giant, one would believe Ymir was evil, and Odin, officially a hero, just happened to kill him for no apparent reason. He never did anything evil, actually just gave birth to people, whose descendants would turn out Always Chaotic Evil, while getting nourished by a cow.
Ensemble Darkhorse: Odin may have been this to the Norse themselves. Because of linguistic reasons some believe that Odin was a less important death-deity, a couple of centuries later and he replaced Tiwaz/Tyr as the dominant god.
Also to the Norse was Freyr, who seems like a footnote in most surviving texts, but was the third most popular deity after Odin and Thor. The three deities are always noted as being the main gods worshiped by the latest original Nordic pagans.
Fridge Logic: Couldn't they chop Loki's head in half without touching his neck?
Apparently not - one version has Loki insisting that by his head, he meant his whole head, so they couldn't leave any of it behind or they'd have forfeited.
Despite his selfish, dishonest, violent, morbid, gray-area behavior, in the more Christianized stories, Odin is portrayed as a benevolent god. And when his infamous cunning, wisdom, and habit of breaking oaths raise questions as to why he's so buddy-buddy with Loki, Odin's made out to be a trusting old fool who only keeps Loki around because he swore a blood oath.
Depends on which ones you're looking at; some Christianized stories snark at him rather a lot. Or literally beat him up: in one story he gets decked three times by a mortal woman. (And then pretends to be a female physician to get a chance to rape her.)
Demoted to Extra: Tyr was originally the king of the gods until Odin got more popular. Now everybody just knows him as "that god with the one hand". This resulted in some versions making the Retcon that Tyr was head god first, but stepped down when he lost his hand to Fenrir.
Odin is just as good as Loki with his constant schemes to delay or thwart Ragnarok completely. In some sources it is implied that he is responsible for every war in human history so that he will have a steady supply of heroes to fill out the ranks of his army in Valhalla so the Gods have a fighting chance at Ragnarok.
Worse than that. The valkyries, universally depicted nowadays as hot amazon babes on winged horses just there to lovingly lift up the spirits of the fallen and take them to Valhalla. Well, some sources describe them as far more than just beautiful Psychopomps for the heroic dead. They helped make your heroes dead. A random arrow deflected in flight hits a chink in your armor. A broken lace on your boot makes you stumble and gives your enemy the opening to strike you down. Etcetera. All the work of the Valkyries, invisibly flitting here and there on the battlefield to screw over the finest of warriors so that Odin would have the best of the best on his side come Ragnarok. It was how the Norse answered the question "why do the good die young while jerk-asses live forever?" It was also why the original steeds of the valkyries weren't beautiful winged horses, they were dark and hoary wolves.
Misaimed Fandom: Unfortunately, due to the Nazi deification of all things blond and blue-eyed, a disproportionate number of the "fans" of Norse mythology you'll find these days are Wotanists, a neo-nazi white supremacist sect who wish to return the lighter-skinned "to their ancestral religion." How these nutters would react to the fact that Norse mythology is heavily connected to other Proto-Indo-European religions, with such concepts as a world tree and serpent being observed as far as India and Persia.
The concept of Valhalla ans Sessrumnir. People often talk about how they eat at Odin's table and are served mead by hot Action Girls. But when they are not eating they are fighting. Maiming each other without dying and regenerating. Forever, or at least until Ragnarök. Drinking with Odin sounds more like making yourself numb with alcohol to deal with the horror.
Moral Event Horizon: Loki crosses it when he orchestrates the events leading up to Baldur's death and prevents his resurrection by not weeping for him. The rest of the gods were pissed.
Never Live It Down: There's something about a guy getting impregnated by a horse that people just don't forget.
Tear Jerker: Loki's eventual punishment and the agony he goes through. (Made worse by the fact that, originally, it was an unnamed woman who didn't cry for Baldur.)
Also, there's something so sad about him having his lips sewn shut and all the others laughing at him. Then again, even before he was turned into a Satan expy, he was still kind of a dick, and it doesn't seem to hamper him in later myths.
Values Dissonance: Thjalfi's freedom was effectively given away as blood payment after his father (accidentally) slightly inconvenienced a God. It says a lot of the times that this was treated as fair, and that the arrangement was depicted as working out happily.
Nowadays, chaining one up, throwing another in the ocean, and banishing the third to the Netherworld might not seem like a nice way for Odin to treat his friend's kids. Even worse, Loki apparently does nothing to protect his children.