There are a lot of changes between the editions. Major differences include:
Nobilis 2e doesn't really have a system for resolving mundane actions, beyond the fact that they automatically lose against miraculous actions. Nobilis 3e has a full resolution system, where characters have (usually) 8 points worth of freeform Skills and Passions, which are combined with an expendable resource called Will in order to do things. Like in 2e, mundane actions can never overcome the force of a miracle, but they can work around it.
The two editions have different sets of miraculous attributes. 2e uses Aspect (physical and mental force), Domain (control over the substance of a Noble's Estate), Realm (control over the Noble's chancel) and Spirit (resistance to direct miracles and use of rites). 3e keeps Aspect and Domain, removes Realm and Spirit and adds two new Attributes: Persona (control over the metaphorical properties of a Noble's Estate) and Treasure (wielding Anchors to generate mundane and miraculous effects).
(Strictly speaking Realm isn't gone, it's just now considered to be a special case of Domain, and is optional for characters.)
Bonds, Limits and Restrictions versus Bonds and Afflictions
In 2e, a Bond was a point of weakness on the character which could be attacked using a Flower Rite, but had no other mechanical effect, a Limit was a permanent weakness which granted permanently higher MP and a Restriction restricted a character's behaviour in exchange for MP whenever it came up.
This system has been completely reworked and unified into a single set of Bonds and Afflictions. Each character has 13+ points worth of Bonds and Afflictions, which help define the character. A Bond or Affliction reflects a truth about a character, and may be a strength ("I'm the fastest man in the world"), a weakness ("I am repelled by salt and iron"), a relationship ("I'm in love the Deceiver Iolithae Septimian"), a personal belief ("I must protect the weak and helpless") or something more complicated. In any case, a Bond or Affliction has some basic effects: whenever it gets it bearer in trouble it generates MP for them, and it can strengthen the bearer's actions if it's a Bond or generate miracles in its own right if it's an Affliction. The primary difference between Bonds and Afflictions is that Bonds are under the control of the player, while Afflictions are under the control of the HG.
An Anchor is now a person, place or thing with which the character has a strong emotional or philosophical connection. Anchors are "wielded" with the Treasure attribute.
Character creation is partly unchanged - Attributes and Gifts cost the same, and characters are still built on the same number of character points. One difference is that each player also builds an Avatar Diagram for their character, which is a sort of free association flow-chart. Chancel and Imperator generation is simplified - each Chancel just has a short list of properties which act as Bonds, and an Imperator has a similar flow chart diagram to a character's Avatar Diagram, made collectively by the players.
Miracles, Miraculous Conflict and Wounds
The basics are the same - when two miracles come into conflict, the stronger (higher level) effect wins out. One major change is that Nobles are no longer immune to direct miracles - a Noble is theoretically as vulnerable to a hostile miracle as an ordinary human. However, Nobles are capable of using their Wounds to avoid the effects of said miracles and can even gain power from them - mechanically, a Wound creates a new Affliction.
There are rules for Imperial Miracles, and player characters will occasionally have access to them.
There are more mechanical parts, like the introduction of Miraculous Edge (which raises the effective level of a miracle) and the Auctoritas Magister (a temporary defense against all miracles, similar to the old Spirit attribute) and action limits (a character is only capable of performing two actions at a time). Also Penetration has been renamed Strike.
Story and Writing Differences
There are a number of changes to the cosmology. These are retcons, rather than in-universe developments.
- The Locust Court is now a place of healing under the jurisdiction of Surolam.
- The Dark and Hell are more sympathetic:
- The Dark has gone from a hatred of humanity to a celebration of human freedom and choice. It's still not nice, since it considers suicide to be one of the most important choices a human is capable of and celebrates it accordingly, but Dark-aligned Nobles aren't necessarily complete monsters either. This contrasts with the Song of the Light which seeks to protect humanity, even from itself if necessary.
- The Code/Song of Hell is one of universal love and compassion, even for things which are wicked, cruel and evil. This contrasts with the more judgemental, demanding Song of Heaven. The text points out that Devils / Fallen Angels are some of the most compassionate beings in Creation, but that in loving things which are corrupt, often become corrupt in themselves. Once again, a Hell-aligned Noble may be a monster, but it's no longer required.
- The Mythic World is the "true" version of the world, and the Prosaic World only exists because of the Earth's trauma.
- The world is more explicitly our world, compared to 2e, which was set in a "darker, edgier" world.
- There are dangerous, powerful things called Actuals, which come into existence due to paradoxes and other acts of Noble hubris. They're sort of metaphysical versions of the Blob: they hunger for meaning and incorporate things of the world into themselves.
- The overall tone of 3e is more whimsical than 2e. It's no less wondrous, but a bit less melodramatic.