I know it is pretty late for this, since the series ended in 2012. But since I just discovered the series last year, I can't help it. I watched the whole series, but I have mixed feelings about it. I love some parts of it but I also dislike some other things.
Especially I find the series finale to be quite unsatisfactory, but that's not the only thing I didn't like. The premise and the characters were interesting and and the cast was very good, however I didn't like what the writers did with them. Many things we were told instead of shown, pivotal things that happened offscreen and an erratic and forced plot with complete lack of internal coherence.
We were told from the beginning that Arthur's and Merlin's destiny is to unite Albion and to bring a golden age where magical creatures can peacefully coexist with non-magical ones. However this never happens, and instead we have Arthur dying prematurely and Merlin only revealing him the truth of his magic hours before of his death. So they never form a proper partnership either. This clearly looks like a huge failure of Merlin's destiny. However, Kilgharrah says that it isn't, that all it's fine because Arthur and Merlin will be remembered as a great king and a great wizard (quite weird statement, since nobody knows of Merlin's abilities at this point and Arthur as a King didn't do anything extraordinary yet), and also at some point in a time of big need of Albion, Arthur may rise again. And Merlin, who unexplainedly is now immortal, is seen, in the closing scene, as an old man still waiting for Arthur next to Avalon lake in our time. So it seems as if writers forgot their own premise, or changed it along the way, and want to force us to accept everything is all right through those lame words they put in the mouth of Kilgharrah.
There is a story about a woman who wanted to preserve forever the happiness and the innocence of her son. So she went to a witch, who told her what she had to do to get her goal. So every day the mother put a dove's bleeding heart in her son's chest and a particular flower in his forehead, so they would erase the effect of time, and her son could be a happy and innocent child forever. So she did until the day he was destined to die. That day she couldn't find a dove or the flower needed, and found his son had suddenly became an old man who was dying. He said he hated her because she had stolen him his entire life, all the experiences, all his dignity. I feel what unintentionally Merlin did to Arthur was something of this sort. He overprotected and lied to Arthur all the time, preventing Arthur to really develop to its own potential. I mean, Merlin hid a lot of important information to Arthur, not only about his own magic, but also about his true origins, and he even advised him against embracing the Old Religion when they had to deal with the Disir.
From the beginning on the series I had a problem, that I kind of liked better Morgana's character than some of the good characters, like Gaius, which to me seemed pretty much as an hypocritical collaborationist in Uther's purge against magic people. I knew Morgana was supposed to be an enemy of Arthur and Merlin in the legend from the first day, but I don't feel her arc into darkness was well developed at all. Also it bothered me that the writers seemed to think that wishing Uther's death was somehow unredeemable evil, despite his awful genocidal crimes. I mean, we even have Gwen taking care of ill Uther at some point, the same Uther who killed her father because of alleged sorcery. To me it seems pretty unfair to Gwen to do this, I don't think Uther deserves at all Gwen's compassion at this point.
Anyway, in seasons 1 and 2 we have a brave, compassionate, and principled Morgana, that turns into a cold and ruthless Morgana in season 3, without any detail about how she got there. Her development happens instead offscreen, in her year outside of Camelot, before she is found and returns. Also later, between season 4 and 5 we have Morgana spending 2-3 years in Sarrum's dungeons with Aithusa offscreen, and again, we are not privy of any detail about what happened there.
Aithusa is pretty much a paradox in the narrative. Merlin hatches her egg. Kilgharrah says she bodes well for Albion, Arthur and Merlin. However Aithusa saves Morgana's life (we are never explained why), sides with Morgana, and ultimately forges the sword that Mordred uses to kill Arthur. Also we never learn how Morgana's alliance with Agravaine originated.
Mordred is another character with a bad development arc. We had this kid in the early seasons which seemed to be a very powerful sorcerer, who swears he will revenge against Merlin. However, when older Mordred appears in season 5, all of this is forgotten, and despite Mordred is still a sorcerer, he seems to be average skilled, nothing especially powerful.
The whole issue of magic is pretty much ill defined. We don't know how magic works, what are the limits of magic, we are not even explained how Uther managed to kill skilled sorcerers who probably had enough power to defeat him.
We are told Merlin is the most powerful sorcerer of history, but we are never shown what that really means. Also Morgana, Morgause and Nimueh are supposed to be priestess of the old religion, but again, we never learn what the old religion is really about, or what are the implications about being a priestess. The whole Emrys thing is never explained, we never learn what the druids expect of Merlin, or how their Emrys prophecies are worded. Also it is never explained why Mordred knew about Merlin being Emrys just by meeting him but neither Morgana or Morgause could find it on their own. Morgana is supposed to be a seer, however we see very little of Morgana using her gift in her own advantage (other than have a few nasty and uncontrolled nightmares).
I liked better the Merlin from early seasons, who, despite being a bit naive, had this rebellious way of thinking on his own, much better than the older and bitter Merlin of later seasons, who became a dogmatic prophecy believer. Also I liked Gwen's intelligence in early seasons, but sadly her character became more and more sidelined in later seasons.
The relationship between Arthur and Merlin developed in early seasons, but later it got stagnated, with Arthur and the knights still bullying and underestimating Merlin until the very end of the series. I know it allowed a lot of slapstick humour, but if we are supposed to expect Merlin and Arthur to follow the Dragon's prophecy, at some point they should consider each other as equals, and work as a real team (instead of Merlin saving Arthur's ass without him never knowing about it), but alas, this never happens.
I would love to exchange thoughts with you on this issues.
I apologize for my English, since it is not my native tongue.