As an example of what Jason is saying, I can offer the following. I have run a BRP swords and sorcery game for years with a magic system based specifically on the Magic World booklet. Some of the limiting factors I used were: having a requirement of POW 16 to be able to use magic (mageborn), basing resistance attacks on current POW (the more you use, the less effective many spells become), sticking to the 'untrained' rule (non-mages can only cast one quarter of INT level spells, rounded down). In addition I made a houserule that required knowledge of other magic disciplines to cast some spells so that the more effective the spell, the more had to be invested in terms of experience and skills. For instance, the Wards spell is a spell that allows a defensive field to be set up that warns against intruders and blasts them in a preset area if they intrude there. The material component is a set of enchanted stones that are set out to define the area. I just required a certain expertise in Enchantment as well as Sorcery to use the spell.
One thing that is really neat about this system is that you can have spells of a defensive nature that a hostile mage must then overcome to harm you...a fighter is not necessarily going to be toasted by a spell directed at him (assuming he has learned some of those defensive spells, like Countermagic and Protection). It's very flexible and flavorful, very much like a D&D style magic system with more options. A PC can have a few spells and a mage can have any weapons, but neither is as good at the secondary ability as the one trained in it. Using it is sort of like RQ without the cultural flavor, that you can tailor to taste.
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