For the most part, Albion modifies stuff already present in D&D, or stuff that was seen in some form or another in D&D settings before: the law/chaos axis for example. There are guidelines for modifying the standard spell-list to make it grittier (mainly by cutting down on flashy damage-based spells, and getting rid of resurrection of any kind), which again is not really new. But how it is put together is something that particularly flavors the setting, especially when you're talking about Chaos and chaos-cults, demons and demon-summoning (the one magical mechanic that is really new in the game) and mutations.
Regarding Chaos worship, some people have asked just what a Chaos-cult in Albion should look like. Now the thing is, the particular manifestations of Chaos-worship in the setting ought to be extremely varied. Law is One, Chaos is Legion. So I don't think there's a lot of uniform things you can say about it. There are heretical subversive cults that appear to worship the Unconquered Sun (like the Cathari), there are secret 'satanic'-style sects, lone would-be witches venerating some dark thing in the woods, there was Bishop Peacock's sect that looked like a regular church congregation on the outside but they were preparing to open a gate to hell in the old Arcadian catacombs, there are of course the Frogmen and their Frog-Gods. So the answer is that the main thing a GM should try to do with Chaos-cults is not make any two look too similar to one another; this is of course a great detail if you choose to focus on a game of chaos-hunters (for example, of a party being sponsored by the Clerical order). Even so, there's a good list of references from the details in the gazetteer and chronology as to provide a GM with ideas of what Chaos cults can look like.
With demons, the whole system for demon-summoning, and the details thereof are explicit in the game. The system is based on the actual medieval ideas on demon-summoning, from grimoires like the Goetia. Demons come in different ranks (corresponding to their level of power), that human beings have labelled as though they were mortal titles: knights, counts, dukes, princes, etc.
The medieval magicians did not believe that summoning demons was necessarily an evil act; if you could control and bind them in the name of divine forces (in this case, the Unconquered Sun), then evil could be used for good purposes (naturally, non-magicians would have a hard time being able to tell if that was the case and would be highly paranoid about the whole business regardless). Suitably bound, they could then be obliged to offer a service or deal with a problem in exchange for being released back from whence they came. But it is also possible to end up making a pact with a demon, if one was inclined to the service of Chaos, where the demon would provide certain gifts in exchange for the magician's loyalty. I did write a little about the kind of gifts Demons can offer: the permanent use of any of that demon's special powers, magic items, access to spell knowledge for magic-users, etc. But again, I didn't want to get too specific here so as not to be restrictive. In short, what is offered will be defined by what the magician wants, and what the demon has the power to give (and yes, there are clear guidelines as to what any given demon is capable of doing).
As far as mutation, here is something that touches a bit on the very early roots of Dark Albion as something that took a bit of inspiration from WFRP. Chaos is inherently corrupting, and people (innocent or guilty) that intermingle too closely with the forces of Chaos run a risk of being forever changed. In some cases, mutations can be intentional on the part of the recipient; something one acquires for power or as a sign of devotion to their demon masters. In other cases, an innocent may be the victim of accidental or intentional mutation, from being at the wrong place and time or having mutation inflicted upon them by a demon or artifact by a cultist. I did come very close to doing a random mutation table and then ended up forgetting about it as we were trying to get the project done. It will almost certainly be the subject of a future blog post of mine and maybe a free download on DOM's website or something (or something for him to add to one of the future Dark Albion books he may choose to write).
Currently Smoking: Blatter Diplomat + C&D's Crowley's Best