This is the rule that defines a Gold token, like how it has a brand-new subtype and a mana ability and 79 electrons. Keep in mind that Gold tokens boil around 3,000 °C even though this isn't written in the rules, so try to keep them out of direct sunlight if you're playing in Arizona.
Among the lists of subtypes, we've got a lot of new stuff, albeit less than Throne of Eldraine had. Demigod and Tentacle join the list of creature types, despite a lack of tentacled Demigods in this set. Gold joins the list of artifact types. And Calix joins the list of planeswalker types.
In the previous set, I added this rule to give some clarity on how a few "consider only these other characteristics" rules worked for legality of casting spells. It says that the object takes on those characteristics while considering legality, which means that continuous effects that affect an object with those characteristics apply to it as part of that consideration. Enough people had trouble catching that extra detail there that I decided it was worth including explicitly in the rule; this isn't a change to the underlying behavior, just a clarification.
Now this, this one's a change! While working with adventurer cards, we examined them next to split cards and morph cards and were okay with how they all lined up. Bestow cards, however, were sitting off to the side, but they want to live in the same space in terms of how you determine if it's legal to cast the card as an Aura. Now they do! This is only a functional change in some very deep edge cases, as the existing rules about how to consider casting-time choices that change characteristics of the spell made almost all the interactions work the way they still do.
There should be a new rule about a new keyword here, but it's missing. How strange.
In games with shared-team turns (usually Two-Headed Giant games) multiple players share an upkeep. Previously, we said that if something triggered "at the beginning of each opponent's upkeep," that ability would trigger multiple times. After all, if the ability went on to refer to "that player," it'd be pretty weird if it just did that once. Cards like Bottled Cloister that did not refer to "that player" got to be pretty weird instead. This new rule supports the multiple triggering when the triggered ability goes on to refer to "that player," but in a change of functionality, these abilities no longer trigger multiple times if they don't refer to "that player" (or similar). Now things are just pretty and not weird.