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.
Acceptable Targets: Many Norsemen view Christians as this, and most Christians feel the same way about Pagans. There are a few exceptions on both sides, however.
Anti-Climax Boss: Lord Athelwulf, King Aelle's brother. He's called in by Aelle specifically to defend Northumbria from Ragnar, is praised as a battle commander, and does show some competence as a tactician when he reigns in a few leeroys on his team. The problem is that he completely fails to assign sentries at his camp, so his men are slaughtered in the night, and when he awakens, he spends a good two minutes praying before battle. By the time he's done, his men are soundly defeated, and he doesn't even make it out of his tent - Floki brings the thing down on him, capturing him without a fight.
Earl Haraldson; in episodes 1-5, he displays extreme cruelty and pettiness. Among other things, he has a man unjustly executed and desecrates his body over a personal grudge, has one of his men killed for trying to sleep with his wife after he himself gives permission for it to happen, has his henchmen kill a thirteen year old boy, attempts to assassinate the protagonists (succeeding in killing at least one of them), launches a full on raid on Ragnar's village, slaughtering dozens just to get one man, and brutally tortures Rollo. However his death scene is so epic and so well acted that it managed to shift many people's opinion of him to Anti-Villain.
Ragnar and Athelstan. When Ragnar first finds Athelstan, he pushes him up against a wall and gets in real close. A few moments later Rollo walks in and dejectedly comments how there aren't any women in the monastery, and the group looks to Athelstan. Later, Ragnar invites Athelstan to have a threesome. And after the timeskip, his reactions to Athelstan's supposed death, Seer-prophesized survival, and arm ring are rather striking and emotional as well.
And then there's the scene where Ragnar and Ecbert discuss a possible alliance while in the bathtub together, particularly the part when Ecbert stares at Ragnar as he undresses.
Earl Haraldsan kicks plenty of dogs, but he's also spent quite a lot of time mourning the horrific deaths of his sons and heirs.
To some, Jarl Borg can come off as either this or a Designated Villain. There's no question that he's an underhanded and ruthless warrior, but that really doesn't make him any worse than the protagonists. Horik dislikes him because of a long-standing dispute over land between them, but it's impossible to tell who was in the right on that one. Ragnar hates him because of his invasion and brief occupation of Kattegat. However, that assault was only in response to King Horik backing out of their alliance at the last possible moment, for petty reasons. After Ragnar takes back his home and apparently forgives him, Borg swiftly admits that Ragnar is the better person, and genuinely seems open to making amends. However, this was all a cruel ploy on the part of Ragnar, who captures him and carves the Blood Eagle on him. These factors, combined with his tragic backstory and Face Death with Dignity scene, can make it seem like all the hate he gets from the protagonists and portions of the fandom isn't completely merited.
Like You Would Really Do It: Trailers for Season 2 seemed to imply that Floki would be killed in battle by Rollo. Given the character's popularity, many fans suspected that this wouldn't be the case.
Magnificent Bastard: Ragnar for his balls in his westward raid and then for manipulating and outwitting the local earl afterwards.
Narm: Kwenthrith's obsession with sex and tendency to hit on anything with a penis can come off as unintentionally goofy and over the top.
One-Scene Wonder: Tostig appears in several scenes in two episodes, but has little more than 5 minutes of screentime. Still many consider him one of the most memorable characters in the entire show.
OT 3: Athelstan/Ragnar/Lagertha. This is almost canon.
Tear Jerker: Ragnar's farewell to Gyda in Brother's War.
If comments from YouTube and various social media are any indication, Aslaug seems to be widely disliked by the fandom for destroying the popular Ragnar/Lagertha relationship. Doubly amusing when you realize Aslaug is the actual protagonist of the saga the show is based on. Less amusing when you realize that she is supposed to be a smart warrior princess in the sage while in the show, the "smart" part is reduced to having visions and the "warrior" part is non-existent - yet.
While Horik, being Ragnar's rival, was never designed to be popular in the first place, he might currently be the most hated character on the show. He was already seen as the root of everything which went wrong in Season 2 for the protagonists, but when he called Athelstan a "worthless individual", believing him to be dead, fans started to declare that the blood eagle would be too good for him.
Too Cool to Live: Let's see, Erik... Badass warrior? Beard of a grizzly bear? Has one-hit kill as a default? Cool voice? Played by Vladimir Kullich? Got top-billing (in the guest starring part of the ending credits anyway)? Nope. Sorry. You're so awesome, you might overshadow the main character. So... off to Odin's Hall with you.
True Neutral: The Seer offers prophecies and guidance to whoever seeks him out, regardless of their allegiances.
We Hardly Knew Ye: Erik (Vladimir Kulich) dies in the fourth episode. You may recognize him from previous badass Viking roles like Buliwyf and Ulfric Stormcloak, or just generally badass roles like the Swede from Smoking Aces.