"I've often thought that in the hereafter of our lives, when I owe no more to the future and being a part of future memory... I can be just a man... and we might meet. You'd come to me, claim me yours, know that I am your husband. It is a dream I have."
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 Le 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.
Being based on the Malory tale, the film is an chivalric romance and has major elements of later Romanticism; it understandably and unashamedly yearns for the nobility and virtue of Arthur's court, and explicitly states that Arthur serves as an example from the past to future memory; though it leaves open the chance that one day humanity may once again return to Camelot's height.
Excalibur provides examples of:
24-Hour Armor: To an almost crazy degree; the suits of full plate mail are worn during feasts and even during sex.
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:
The sword in the stone/Excalibur (in Morte d'Arthur, the two were different swords)
Aluminum Christmas Trees: Find it unusual that Morgana sleeps in the same room as her mother, despite being four or five years old? Up until the 1800s, it was quite usual in Europe for entire families to sleep in the same bedroom, if a house even had divided rooms at all.
All-Star Cast: Patrick Stewart, Liam Neeson, Gabriel Byrne, Helen Mirren, Nicol Williamson, and Ciaran Hinds. This is actually an inversion as none of Them were especially big at the time, at least not in the United States, and Boorman chose Them because They were relatively unknown and didn't want big stars to overshadow the production.
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.
Awesome Moment of Crowning: Arthur's literal crowning is not shown, but he instead gets an awesome moment of Knighting done by a rebel warlord with Excalibur, after which the rebels submit to him as their King.
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.
Dawson Casting: Nigel Terry was 35 playing Arthur from a teenager to a 20-to-30 something King and then finally as an old man. Similarly, 29-year old Cherie Lunghi played Guinevere from teenager to full adult Queen to an old woman living out the rest of her life in quiet misery as a nun.
The Dragon: Mordred is The Dragon in the classic sense, while The Dragon is, in the movie, a never seen but omnipresent Nature Spirit.
Lancelot could be seen as a heroic version for King Arthur and the Knights of the Round Table.
The Dulcinea Effect: Perceval, unarmed squire, wanting to champion Guinevere against a fully-armed, battle-hardened Sir Gawain.
End of an Age: The final minutes drive this home with a hammer, but leave open a glimmer of hope that mankind can once again reach its own Camelot; Arthur is taken off by three queens into the sunset, Excalibur is lost, and Magic is dying or already dead; however Perceval survives to tell Arthur's legend so as to serve as an example, Arthur makes a prophecy that a new King will come and the sword will rise again, and Merlin still lives in our dreams.
Good Old Ways: Arthur's plan to win the final battle, since they have the aid of the Dragon's breath for one last time: "Back to the old ways! Speed of horse!"
Groin Attack: Arthur appears to do this to Lancelot at one point during their duel, complete with making Lancelot go "OOH!" in response. Later during Lancelot's nightmare of fighting his armor, his armor appears to be trying to castrate him, swinging the sword at his family jewels.
Impaled with Extreme Prejudice: Mordred runs Arthur through with his spear, but Arthur kills him with his sword. In fact, this is the reverse of what happens in Le Morte D'Arthur, so that Arthur strikes his last blow with Excalibur.
The Duke of Cornwall and some other knights also die this way.
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.
Karma Houdini: The knights who killed Uther aren't punished for it.
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.
The Lancer: Kay, Lancelot (naturally) and even Perceval all play this role to Arthur at some point during the movie.
Nice Job Breaking It, Hero: When Guinevere is accused of adultery with Lancelot (at that point she was faithful, in body if not mind), Arthur puts her on trial to demonstrate that no one in the land is above the laws. Guinevere is upset at Arthur's willingness to put his duty as a king above his love for her, is humiliated in front of the entire court when no one (sans Perceval) will fight for her honour, and then watches as Lancelot comes to her rescue and is badly injured in her defence. Naturally, the whole thing results in nothing but Guinevere falling (more) in love with Lancelot and deciding to consummate her relationship with him. Arthur's sense of duty might have been impeachable, but it would have saved them all a lot of hardship if he'd just stood up for his wife in the first place.
That's not even mentioning the fact that up until this point, Lancelot wasn't even in the kingdom. He had been deliberately avoiding Camelot in the hopes of staying away from Guinevere. Naturally Arthur's trial of Guinevere brings him racing straight back again.
During his fight with Lancelot a frustrated Arthur literally breaks the unbreakable sword by using its power to defeat Lancelot, who up to that point had been dominating the fight, shattering it as he cuts through Lancelot's sword and then slams it right into his armor, knocking him down and almost killing him in the process. Fortunately when Arthur admits out loud that he let the sin of Pride get the better of him the Lady of the Lake comes along, fixes Excalibur and gives it back to him, upon which Arthur silently vows never to abuse the sword's power again and then hires Lancelot to be his right hand man.
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.
Rule of Cool: Mordred's golden armor — and generally, knights wearing their plate armors outside of battle whenever they can.
Screw This, I'm Outta Here!: Merlin abandons Uther to his traitors. "Merliiiiin! Where are you? Call your dragon... Weave a mist... to hide us..."
Self Healing Phlebotinum: Arthur breaks Excalibur by using it to satisfy his ego and defeat Lancelot. Upon realising what he has done, he hurls it into a lake and repents, upon which it is returned to him in perfect condition.
Self-Made Orphan: Mordred kills both his parents, though he doesn't outlive his father.
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 latter, 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.
Spared by the Adaptation: Perceval. In some versions of the legends including Malory's, he dies upon fulfilling the Grail Quest and never returns to Camelot. Here he's combined with Bedevere, the Sole Survivor of Arthur's last battle.
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.
"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!"
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.
What Happened to the Mouse?: A consequence of having Loads and Loads of Characters in a two-hours movie. Igraine is a notable example of this, since you could at least assume that many of the knights that go missing were slain in battle. The last we see of Igraine however, is her screaming: "I want my baby!" and urging Uther to go after Merlin. (Though she disappears at that point in the legends too).
A very unfortunate example is Arthur's adopted brother Sir Kay, who despite remaining steadfastedly loyal disappears in the final battle. The last we see is him fighting back to back with Arthur and Perceval; then Lancelot appears and Kay dissappears. It's jarring as Kay was clearly steadfast in loyalty and love for Arthur throughout the film, and all of a sudden he's gone without comment. (Of course he died, but he doesn't get a death scene of his own like others do.) This is in keeping with both Le Morte d'Arthur and The Once and Future King, where he appears as major character early on and then just kinda... goes away, although Kay is in general a steadfast and decent fellow in this film, rather than the boorish jerk he is in other sources.
Sir Kay does get a "pre-death scene" : a conversation with Arthur in which he reveals that Merlin appears to a him in his dreams and told him he would fight valiantly for his brother, strongly implying he while die in battle.