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.
Colbert Bump: It's received a lot more attention lately after being revealed as the favorite musical (and movie) of Lewis Lovhaug, the host of the wildly popular Atop the Fourth Wall, with him citing the important role it played in forming his personal sense of morality as a child.
Harsher in Hindsight: Used in-story. Guinevere, in the beginning, seems flattered by the idea of her being the subject of a war. Later...
Designated Hero: Arthur is a Spoiled Brat who is introduced to us while he's sleeping with his brother's girlfriend. He becomes king primarily because he's Uther's son, rather than any merit on his own part. His character develops a little over the episodes but, as seen below, he ended the season pretty much as unpopular as he started.
Draco in Leather Pants: Morgan is a rare female example of this trope, mainly due to her charisma and that she was played by the gorgeous Eva Green. It's no wonder that much of the audience was rooting for her, particularly since she was pitted against a rather gormless version of Arthur. (See The Scrappy entry below).
Ensemble Darkhorse: Declarations of "Team Morgan!" are not uncommon. Merlin is also popular. Among the knights, Gawain for being the most badass. His actor Clive Standen later got more recognition as the also-badass Rollo in The History Channel's Vikings.
Nightmare Fuel: Morgan's dying scene. Man, one doesn't wish such an experience even for a villain.
Romantic Plot Tumour: Arthur/Guinevere and the consequent Love Triangle with Leontes was generally considered boring and shallow, particularly since Arthur/Guinevere's attraction was based almost solely on lust (occurring on the basis of one dream, two short conversations, and sex in a cave on her wedding day to another man).
Foe Yay: Morgan and Merlin. Morgan and Arthur have some too.
The Scrappy: Four episodes in, and there's already an overwhelming number of viewers who consider Arthur himself to be this. It's early, so only time will tell if he can be Rescued from the Scrappy Heap.
As of the end of the first season, he's still not very popular.
And since it turned out to be the only season, he never will be.