The movie covers a rather long span of time (60 years, at the very least) and thus, as Boorman put it, focuses on the story rather than on the characters. It can thus roughly be divided into five partially overlapping parts: the first part follows Uther Pendragon, the second follows Arthur, the third follows Lancelot, the fourth follows Perceval, and the last goes back to Arthur.
The source material (mostly Malory's Morte d'Arthur) is treated in a very syncretist kind of way, merging many characters, events and elements. This arguably allows the movie to display many more Arthurian motifs than would have been possible to show in a two-hour movie by staying truer to the original story, all while cleverly avoiding the Compressed Adaptation effect.
Adaptation Distillation: The film sometimes takes liberties with the original legend (many of these alterations became canon in popular culture, and have later been re-used in other Arthurian movies), and as already mentioned, merges together many characters and elements of the Arthurian tales, most notably:
Anachronism Stew: Justified, as Boorman intentionally took an ahistorical and mythical approach with his portrayal of the time period.
Armor Is Useless: Averted, at least some of the time; fully armored knights in the movie often have to really pound the hell out of each other to have any effect.
Arthur: I was not born to live a man's life, but to be the stuff of future memory.
Bed Trick: Used twice — but magic is involved in both cases.
Bilingual Bonus: The Charm of Making is Old Irish for "serpent's breath, charm of death and life, thy omen of making".
Bittersweet Ending: almost every major character is dead by the end of the movie. But the Land has been replenished, and England (and the world) has a Legend built on the heroics of King Arthur and his knights.
Breast Plate: Morgana's armor, which looks like a sheet-iron corset with nothing under it.
Interplay of Sex and Violence: Truces and friendships are broken, wars and plagues are started because of sex. The slow death of the Duke of Cornwall, impaled on a row of spears, interspersed with the scene of Uther, disguised as Cornwall, having sex with Igraine, is quite iconic.
Knight in Shining Armor: Averted at the beginning of the movie, where the armors are more dark and matte than shiny, then played straight after the first encounter with Lancelot, but they do get rusty later.
Another facet of the Fisher King aspect. The better Arthur's doing, the shinier the armor.
No Indoor Voice: Pretty much everyone in the entire movie. Justified since they spend a tiny portion of their time indoors.
Offscreen Teleportation: Merlin — justified, given his nature, and also subverted in a scene where we see him walking toward the camera, from a distant background, while other characters talk in the foreground, having not noticed him. He reaches them just as one of them asks "And who is Merlin?"; he also does onscreen teleportations.
Shout Out: Several, notably (and surprisingly) to Star Wars(just watch the first sequence between Arthur and Merlin in the woods, or the fight between Lancelot and his Enemy Without), and (less surprisingly) to The Lord of the Rings (as Boorman's original project was indeed to adapt the later, before he switched to Arthurian myths:
Arthur: Merlin. Your wisdom has forged this ring. Hereafter, so that we remember our bonds, we shall always come together in a circle to hear and tell of deeds good and brave. I will build a round table where this fellowship shall meet.
Sociopathic Hero: Uther, arguably — subverted in that he eventually admits that he is tired of wars and battles.
Soundtrack Dissonance: The lyrics to O Fortuna are about how fate is capricious and thus cruel, but the song is treated as something far more uplifting.
Stealth Hi/Bye: Both played straight and subverted with Merlin. In one scene the audience sees Merlin approaching but the characters don't, and when Arthur says, "Who is Merlin?", previously-unnoticed Merlin steps up and says, "I am Merlin." In a later scene Merlin says, "The time has come for me to go," then turns to leave. Normally one would expect Merlin to just vanish, but Arthur instead starts following him and asks where he's going.
The subversion happens when Arthur storms into a castle which is already being stormed, precisely in order to stop said storming.
Suspiciously Apropos Music: The recurring music illustrating the impossible love between Lancelot and Guinevere is Richard Wagner's Prelude to Tristan und Isolde; Perceval finds the Grail while Wagner's Parsifal Overture is playing; and Siegfried's Funeral March (by Wagner) plays while Arthur is transported to Avalon.
"It is everywhere. It is everything. Its scales glisten in the bark of trees. Its roar is heard in the wind. And its forked tongue strikes like... *lightning strikes* like lightning... yes, that's it!"