King John swears on his mother's life that he will sign the charter (Magna Carta). However, this oath loses a lot of its importance when you realize that King John DESPISES his mother and probably wouldn't hesitate to have her killed given the chance.
The '73 Film
During the lines "Reminiscin' this and that and havin' such a good time" in "Oo-De-Lally", Robin Hood and Little John are each urging the other to go first when crossing a river by log. The brilliance? In the original tale, the two meet when Little John prevents Robin Hood from using a log to cross the river; that's what they're remembering.
Also in the 1973 film, Prince John decided to order the execution of Friar Tuck to lure Robin out, a suggestion even Sir Hiss was horrified by. However, there is historical context to this. John and Richard's father, King Henry II, had Thomas à Becket, then Archbishop of Canterbury, assassinated. Where do you think Prince John got the idea?
Sir Hiss' horrified reaction is also historically accurate. At the time, the Church was a powerful political entity and a noble didn't have the right to judge a priest (for whom the canon law was applied). Of course, Hiss knows that. And he knows the dangers of upsetting the Pope.
Also, historically, John was actually excommunicated, and England placed under interdict, by the Pope during his reign.
While Robin is daydreaming while cooking, Little John tries to get his attention by calling increasingly extended forms of his name: first Rob, then Robin, and then Robert, using the French pronunciation (sounds like Ro-bear). It's easy to assume that this is because Little John is a bear in the film and that Ro-bear is a nickname. When you think about it further, the French pronunciation because they live under and speak the language of the Plantagenet dynasty, who are French and the language spoken in England at the time would have been an iteration called Anglo-Norman French.
Not to mention that in the legend Robin's birth name is Robert Fitzooth.
"The Phony King of England" is as close as Disney could get to the real-life Bawdy Song "The Bastard King of England" in a family flick; which is similarly a disparaging song aimed at Prince John.
Also in "The Phony King of England": all the world will sing of an English king a thousands years from now / and not because he passed some law. King John's main (possibly only) achievement was to sign the Magna Charta.
A kid might not realize it at first, but the fact that Alan-a-Dale the Rooster is in jail too is a hilarious fourth wall breaking, the gag being that Prince John was so enraged with putting everybody in jail that for some reason he even managed to pull the narrator in there, even though he isn't part of the story. Seen in that light, the way we first hear his voice saying the beginning of his line, in a typical narrating, and seeing it being ended by a living character on-screen, is hilarious.
The gag is not helped, of course, by the fact that said Alan-a-Dale has already been physically seen earlier, and even warned us at one point that he would be narrating "what happened or what's happening", which already implies that his relationship to the story is closer than that of a normal narrator.
Almost all commoners in the film are domestic or wild animals native to Europe (apart from a few raccoons seen in the background), but Prince John's entourage consists almost entirely of African wildlife apart from the wolves. The brilliance here is that the rhinos, elephants, crocodiles and vultures can be mercenaries that John hired from abroad.
The Rabbit family scrimped and saved to give Skippy a birthday present of one farthing - the smallest denomination of currency at that point. That's like having to save up to give someone a penny nowadays. Now imagine the economic hardship that would create that kind of situation.
From what's implied (can't remember if it was outright said), the reason for their hard economic situation is because Prince John raised the taxes to such a high amount. Slightly lessened, as the farthing was probably worth more than a modern penny (a farthing did go pretty far in those days)
Robin is an outlaw because he shot a deer in the King's forest. In the 1973 film, animals are sentient.
Lessened somewhat by the fact that it's never stated exactly why Robin's an outlaw in the Disney film - one can very easily just assume it's down to constantly managing to annoy Prince John. Heck, they even present it as though becoming an outlaw was more a lifestyle choice than an actual ruling.
Robin and Marian drive off in a carriage. Little John sits at the front with a whip. It is never shown what animals are pulling the carriage, but again, animals are sentient in this universe, and the heroes are making them perform manual labor and whipping them when they go too slow.
Maid Marian's death in Robin Hood was already a punch in the guts, but the real kicker comes when you realize that she died in order to save the life of King Richard. The same King Richard who eventually gets back to England, only to bugger off to Normandy and get killed there, giving Prince John the opportunity to ascend the throne and become King. So Marian died for absolutely nothing.
Similarly, every adaptation of the Robin Hood legend which casts Prince John as the ultimate villain because he's "not the true king" runs into the Foregone Conclusion that he's going to end up being the true king anyway.