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.
In some stories, Gawain is Arthur's best knight and the ideal of chivalry. In others he's a flawed but principled hero. In other ones he's a boorish, impulsive Ax-Crazy Antihero. In others, he's the Table's Boisterous Bruiser, good-natured and hot-blooded, but also dim-witted, impetuous, and a frequent tool in conspirators' plans.
Kay's furious, hot-tempered personality is explained away in Parzival by Wolfram von Eschenbach as a function of his job as Arthur's bouncer. Tons of people showed up to the castle every day claiming to be worthy knights when they really weren't, and it was Kay's job to sort out the bad ones from the good. This explains his really jerkish behavior when Percival arrives at Camelot. If that's the case, he would also be a Mean Brit long before Simon Cowell was born.
Is Mordred a rebellious and treacherous son or victim of fate?
Medieval romancers occasionally note that, though an ally to Arthur, Merlin is actually evil, treacherous and disloyal by nature. He does take several questionable actions over the course of the story.
The character that receives the most alternative character interpretation is Morgan Le Fay due to the inconsistent characterization of her. Is she genuinely driven by wrath towards Uther for having her father killed so that he could have her mother (which in turn begat Arthur)? Or is she a sociopath who simply uses this tragedy as a Freudian Excuse to cause havoc For the Evulz? Is she not evil and/or antagonistic in the first place as some works (e.g. Geoffrey of Monmouth's Vita Merlini) imply? And what is her reason for taking her mortally wounded half-brother, Arthur, to Avalon to be healed?
Americans Hate Tingle: One would have better luck trying to find a penguin in the Sahara than finding any Arthurian lore from Ireland that depicts Arthur in any fashion other than a petty horse thief.
Also worth mentioning Scottish Arthurian literature that depicted Arthur and members of his court as tyrannical and lecherous villains, and portraying Mordred's power grab as a legitimate action to reclaim his birth right.
To a lesser extent, the Scots, who, while unrelated to Arthur directly (they're Irish and Pictish mostly, with a little bit of British down south) got on the bandwagon as well. They do, however, revise the stories from time to time to make it more favorable to their own national heroes (like King Lot).
The Germans love Arthur too, particularly the Grail legends.
Also interesting to mention that there is also one work of Arthurian lore, Melekh Artus, written in Hebrew in 1279. The fact that Jews, who weren't exactly treated all that well during the times most Arthurian works were composed, also seemed to like the Arthurian legend (sans the grail stories) suggests that some ideals of the legends appealed to people outside its intended audience.
Tied into Lancelot, most adaptations will include a carbon copy of the character even if they choose a different name for the character. Some are good. Some are bad. All take time away from King Arthur.
Although it has been speculated that Lancelot may in fact have been the hero of a folk tale of his own that was then absorbed into the Arthurian cycle (not unlike what happened to Tristram).
Recycled Script: The tale of Sir La Cote Male Taile is pretty much the exact same story as the tale of Sir Beaumains; a lowly servant becomes a knight and is given an insulting nickname by Sir Kay, they go on a quest with a damsel who mocks and degrades them endlessly, they wind up proving their worth and changing the damsel's view on them, and their true names are eventually revealed (Sir Beaumains = Sir Gareth and Sir La Cote Male Taile = Sir Breunor.) Both of these stories are told in Le Morte Darthur, so a sense of Deja Vu is inevitable upon reading them one after the other.
Romantic Plot Tumor: The original information on Arthur focused on his ability to destroy Saxon armies whole-sale in twelve battles before dying in Pyrrhic Victory due to base treachery from his rival Mordred. Nowadays, the late romantic subplot of Lancelot dominates almost all retellings of the story, sometimes as the entire plot.
Woolseyism: Originally spelled "Myrddin," but changed to "Merlin" by Geoffrey of Monmouth because Myrddin sounded/looked too much like the French "merde" and he wanted to avoid the Unfortunate Implications of naming his prophetic wizard after the French word for poop. It's worth noting that his audience was French-speaking Normans. In Welsh the -dd would be pronounced as -th.
The film King Arthur
Alternative Character Interpretation: Much of what Cerdic does is open to this. His intentions are hard to read, and he doles out kindness and cruelty in the same gruff and joyless tone of voice.
A good example when he protects a female prisoner from two of his own men, killing one of the men when he objects, on the argument that "we don't mix with these people". Then, when she starts to thank him, he turns around and kills her as well. Was he really motivated by a theory of racial purity? Or did he simply realize that he couldn't protect her for long?
Galahad saying he doesn't kill for pleasure "unlike some", and Tristan replying that he should try it someday because he might get a taste for it is rather funny, considering that Hugh Dancy (Galahad) and Mads Mikkelsen (Tristan) star in Hannibal as Will Graham and Hannibal Lecter, respectively.