: Perfect cast, great acting (especially Colin Morgan as Merlin).
Characters and relationships with plenty of potential 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 (aka: genocide)
Endless harping on the two protagonists' alleged destiny to unite Camelot and liberate magic, neither of which actually happened.
The writers' absolute refusal to have Merlin reveal his magic - which, ultimately, was the reason the whole plot went to hell.
Significant characters belong to one of two groups: those who adore Arthur and those who loathe him. Any relationship that isn't important to Arthur is quickly truncated - either by killing off the character(s) involved or writing them out of the plot. No one seems to have a life/desires/aspirations except in relation to The King.
The writers' clumsy attempts to avoid slash subtext (heaven knows why they didn't notice before
they finalised the premise the homoromantic potential of a story where the protagonist practically lives for another guy), which led to Merlin and Arthur's relationship stagnating in the 'juvenile banter' stage forever, occasionally becoming downright abusive on Arthur's part: Merlin saves Arthur's life; Arthur calls Merlin "the worst manservant ever" (while throwing something heavy at his head for emphasis); Merlin spurns all personal relationships and desires so that he can protect Arthur; rinse, repeat.
The so-called 'Once and Future King' is emotionally immature, self-righteous and oblivious to the last; not a bad guy, per se, but very typical royalty who doesn't deserve his epithet or the devotion all his allies lavish on him.
Merlin's portrayed as an 'incredibly caring' man who nevertheless doesn't give a flying f**k about his fellow magic users (Supporting them would mean opposing Arthur, and we can't have that, now can we?)
The major villains, Morgana and Mordred, are overall more sympathetic than the MC's.
= Why did I watch Merlin
to the end? Stockholm Syndrome is probably the best explanation. My advice: watch the first 2 seasons to become familiar with the premise, then go read the excellent canonverse fanfic on FF.net.