I think because I hate the DnD with so much passion and for so long (13 years now...) the magic system is probably the first thing I was glad to experience in a different fashion.
It''s hard to explain without all my books around, but I am still very much attached to the Simulacres system.
Interestingly, the White Wolf system is very similar in appearance.
In WW - Vampire, you learn a Path of magic, and with increasing experience you can buy more and more powerful powers in that Path. It combines the classic levelling of classic systems of games like DnD with the added effect that you cant just go and buy the best spells for a certain level, you have to learn your art thoroughly. Rituals are a different kind of magick and can be learned separately, provided you have the required skill in a particular Path.
In Simulacres, it was much simpler: every single spell you learn is a separate and unique skill, that can be improved with experience. So when you learn the basic fireball, it''s not so impressive, but if all you do is cast Fireball and increase only that one spell, you are gonna start having some pretty heavy firepower, to the exclusion of any other spell.
I liked the system so much because it really demanded that the player invest his experience carefully.
At character creation, you would buy Energies, a trait that defined your affinity to a particular path of magic. The system being so open and generic, an energy could be very specialised (like Protection), or a bit more general (like Fire).
Spells would be associated to a particular Energy.
So if you wanted to cast the spells to the best of your abilities you would have to invest from the start, which would then mean you would tend to stick with the spells of the Energies you had.
Of course you could later on learn new Energies to increase the spectrum of your spellcasting abilities. Or you could stick with your Energies (it''s more expensive to increase an Energy, much more than learn a new spell) and specialise.
This lead to very specialised, and more importantly, very unique spellcasters.
For instance I had a druid with a "dot" in Protection (so he could walk around unharmed), two "dots" in Healing (he was a wandering healer, a sort of holy man), and two "dots" in Nature (to make trees grow...)
The only spells he had at the start were Heal Wounds, Increase Plant Growth, Shield. Like I said, pretty specialised.
And I am not even gonna start with my favourite magic system of all times (at least in concept, I havent tried it), Mage : The Ascension.
Sancte Isidore ora pro nobis !
-----------------------------Sancte Isidore ora pro nobis !