Verdict:

The two most notable traits in King Arthur are its soundtrack and its villain.
The score comes courtesy of Hans Zimmer and has his usual bombastic grandeur accentuated by a powerful undercurrent of percussion, but I find it to be even better than a lot of his other work. It is catchy, with a good variety in character and pacing. It is epic and emotional, especially the track “Woad to Ruin”. It stands well on its own so you can listen to it on the bus and have an epic experience. This is one of the best soundtracks of all time, sadly attached to one of the most average and forgettable movies, condemning this epic musical achievement to exist in obscurity.
The other notable feature is Stellan Skarsgård, who plays the leader of the Saxons. His acting is subdued, too subdued perhaps, as he whispers his lines a lot, but there is something regal and menacing about his delivery. His spoken lines are few, but weighty. I wish this guy was in the film more. He is only in about 5 scenes. Someone needs to make a movie about that character!
This is where the notable bits end and everything else about this film reeks of averageness. It looks ok. There are a few cool battle scenes, if examined separately from the rest of the film, like a cinematographic demo, but the movie as a whole lacks punch.
The dialogue is not natural. The characters lack definition. They were given different looking outfits and haircuts, but you won’t remember most of them because they aren’t given a backstory.
The point of the movie was to attempt a more historically grounded portrayal of king Arthur’s legend. Well, it gets half-way there. King Arthur looks like a LARP event, with dudes carrying dual swords on their back and wearing cool mustaches. Keira Knightley is out in the snow half-naked shooting arrows.
The film portrays Arthur as a Sarmatian fighting for the Romans, who finds himself on the edge of the Empire in Britain, starts distancing himself from his old masters and goes native, ending up protecting Britain from the Saxon invaders. It is an interesting premise that the movie doesn’t do that much with. The main theme is the conflict of various loyalties. To this end, there is drama and raised voices that ultimately just feel like acting for the acting’s sake. With a premise like that, you would expect an epic adventure. It’s a birth of a nation, after all. Instead, King Arthur comes off as bland and local.
King Arthur is worth a watch for its soundtrack and a few actors, but don’t go out of your way to see it. There is supposedly also a director’s cut of this film that’s a better product. Watch that version if you can.