Read through interview about incoming Arneson documentary, out of curiosity read through his and Richard Snider's "Adventures in Fantasy: Book of Faerry and Magic." Several things struck me as very unusual.
First and biggest of all, the spells are differently arranged in levels of power. In almost all DnD things I've met, regardless how old or new they are, you get Magic Missle or equivalent on first level, Charm Person on first level, some Mage Armor on first level, some kind of Fly on third or forth and so on. Of all different changes I saw, arrangement of power in spell levels is very rarely challenged in DnD: at the best, spells are cut away or complicated but they rarely move drastically at mass from their traditional spell levels. AiF isn't DnD but it had a common root and it is interesting to see a different pattern being set here.
I can only approximate spell levels in AiF because AiF magic system is non-Vancian and uses spell points and not spell levels/slots/memorization (so any mage with knowledge of spell and at least six spell points can cast "Blood Wounds" as many times as they have spell points for it), but as spell points range from 1 to 10, I'd say the error is within one level of spells in terms of relative power.
And what struck me as odd, is that "Charm Person" spell has 4 points so is about level 4 or 5 in "AiF" and is respectively more powerful, akin to Dominate. There is earlier "Charisma" spell that boosts Charisma stat but it makes no other magical effects, so it is more like "Eagle's Splendor" than "Charm Person". There is "Rhetoric" spell which is more like very powerful "Command", but it take two spellpoints and it is way too powerful for even level two; it doesn't pretend to be false friend either.
(And by some reason snakes have a separate Charm spell specifically for them, just as Control Horses spell only controls horses. I don't know if it makes any sense in a setting.)
The general idea is still here, but differences are multiple enough to create a different feeling of world through what magic is available, when and how. As all mages here cast more or less as 3E Sorcerers or even psionics with no cap I can see for how many spells they can know total and when, mages in AiF also learn spells very-very slowly. What prevents players to abuse the system by saying 'I sit in my lair for 6 years learning 20 spells" I don't know but I wonder if it would work better with seasonal adventuring like one in 'Pendragon.'
Second unusual thing is that mages can, potentially, cast multiple spells per round and this is not mechanic I see often, with mainstream DnD/PF going into big pains to assure that even super-duper-mythic Haste doesn't give you two spells in a round. In AiF as mage's experience goes up, their "points may cast per turn" allowance grows as well, and at level 15+ becomes 13 spell points per turn in a system where highest spell points cost is 10.
Which means that the mage can potentially cast in one turn the most powerful spell and also something else. According to the text, each turn is about 3 melee rounds, so while the magic is noticeably slower at the beginning-middle of progression, at very high levels a mage can eventually spell-sling medium-to-high spells faster than once a round, and low-to-medium spells more than once a round from middle levels up.
I suspect those are unbalanced mechanics, but they are at least somewhat offset by third unusual thing: the scarcity of direct damage spells. The earliest direct damage spell "Blood Wounds" takes six spell points to cast and there aren't many direct harm spells at all, mostly on high levels (8-10 points).
Instead magic is more about buffing/debuffing (Might, Strength, Throe), mind- and emotion control spells which often are also about buffing-debuffing (Abandon, Anger and Revenge, three Charm spells, Control Horses, Desire, Ecstasy <which is, curiously, berserker rage>, Empathy, Fascination, Fear, Guile, Hatred, Insanity, Insolence, Inspiration, Mad Fury and Malice <which I think have their power cost messed up>, Mind Control, Obedience, Persecution, Suggestion, Telepathy, Terror, Trance, Truth). There is a relatively many divination spells, a couple of weather/elemental spells which mostly double as high-level attacking spells, one shapechange spell, one illusionary appearance spell, one invisibility, one teleportation, a couple of agricultural spells, several oddly specific protection spells (Protect Dwarves, Protect Warriors, Protect from Chaos, Protect from Dragons), a couple of life manipulation/disease/poison spells, one age-deferral spell, no summoning (elemental summoning is a separate thing), and a couple of healing spells.
Which is a forth unusual thing: mages do heal. I don't know if there are any priests around in "AiF" with better healing, because mage healing is limited to stamina and hitpoints in a system where the same mage has 'save or die in about 6 rounds' Poison/Plague spell, but still, mages can heal. "Cure Stupidity" is, funny enough, a spell to permanently boost Intelligence on somebody else and not a direct cure to anything.
I am finding it refreshing. Even if clerics are somewhere introduced with better healing later on, mages still can save your life in a pinch.
Fifth thing which I'm finding kind of interesting, is that the magic itself works differently in regards of who is casting it. Alignment here is more like allegiance, with Lawful-Neutral-Chaotic sides of some eternal-conflict-not-clear-from-the-text, and spells are divided in four roughly equal categories: 28 non-alignment spells which everybody can use, 17 Lawful spells, 22 Neutral spells and 17 Chaotic spells. Mage chooses their Alignment by learning their first Alignment spell, which makes it entirely possible to go through life without ever making a choice and I have no idea if creators ever taken this into account. Aside of usual thing that Chaos cannot learn Law and otherwise, both Law and Chaos can cast Neutral spells, and Neutral can use one other Alignment, but their effects are halved in both those cases.
What makes it nifty, is that it makes all 'Lesser' versions of spells kind of implemented already from within the spells itself. There is no need to explain, setting-wise, with separate Lesser spell, why this person became a mindless vegetable not for life (as per usual casting of Annihilation spell by Lawful mage) but only for 48 hours as it was cast by Neutral mage, or why Dispel has a cap on a level of effect it is able to work with.
Six and smallest thing, is that Necromancy spell is about talking to dead people. Which isn't divination by innards of a goat, but still close enough.