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.
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.
Erik is incredibly popular with the fandom. Primarily as a result of copious badassery, one-hit kills and a stellar performance by Vladimir Kulich.
Tostig, the Old SoldierDeath Seeker, has become extremely popular as well, for similar reasons as Erik. Not the least of which for his epic speech - a tremendous reading by Angus MacInnes
Floki has captured the hearts and minds of many in the fandom and has been called the the Viking version of the Joker.
Foe Yay/ Ho Yay: Ragnar and Aethelstan. When Ragnar first finds Aethelstan 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 Aethelstan. Later Ragnar invites Aethelstan to have a threesome.
Jerkass Woobie: 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.
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.
OT 3: Athelstan/Ragnar/Lagertha. This is almost canon.
Tear Jerker: Ragnar's farewell to Gyda in "Brother's War".
The Scrappy: 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.
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.
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.