: Great cast, good acting (especially Colin Morgan as Merlin).
Characters and relationships with plenty of room for growth.
A very promising setting.
: Needlessly convoluted plots and a formula that repeats every season; no disturbing the status quo.
Blatant disregard of the horrors caused by Uther's Purge.
Endless harping on the two protagonists' alleged destiny to unite Albion and liberate magic, neither of which actually happened.
The writers' absolute refusal to have Merlin reveal his powers - which, ultimately, was the reason the whole plot went to hell.
The utter lack of respect the writers show towards every character except Arthur (and, by proxy, Gwen) : their relationships fail or lovers/families die, but they're either weirdly uncaring or their grief is irreverently exploited for the sake of the plot; they have no dreams, ambitions, or even much of a life except in relation to Arthur; disliking Arthur, regardless of whether it's justified or not, is a sure-fire ticket to villainisation and death.
Merlin and Arthur's relationship was pivotal to the plot: the great king and his trusted court sorcerer could never exist while Arthur continued to ignore Merlin's advice due to his being, quote, 'a worthless, idiotic servant.' But the creators' bizarre tactic for circumventing slash subtext led to their so-called friendship stagnating forever (Arthur alternates sneering disdain with complete indifference, while Merlin's inexplicably and unwaveringly loyal to the point of being both suicidal and homicidal), dragging the plot down along with it.
Arthur is selfish, immature, short-sighted, self-righteous and oblivious to the last; still practically a saint compared to his father, but not even remotely worthy of being lauded as 'The Once and Future King.'
Merlin's contradictory portrayal as "a nice guy who doesn't give a flying f**k about magic users," wherein he endorses the genocide of his own people
on the word of an exceedingly dubious prophecy. Yet, this unrepentant traitor to both magic and magic users is apparently 'The Greatest Sorcerer of All Time.'
All of this means that most of the antagonists are far more sympathetic than the MC's.
= Why did I watch Merlin
to the end? Stockholm Syndrome is probably the best explanation.