How do you balance magic-users in your campaigns? In the big gold book they are not balanced at all. You either know Magic (I'm speaking about the specific power-type) or you don't. This is not a problem if all players are assumed to know magic or if they are supposed to learn magic during play, but if one player wants to be a mage and the others don't, BRP doesn't really offer any way to justify the difference. Ok, it says that only characters with high enough Power score can learn Sorcery (the other power type), but that is boring in my opinion. Differences in Power already dictate how good you are at using magic. Other than that, the book just says certain professions are more likely than the others to know magic.
I was thinking ways to make any character a potential spellcaster if the player wants to play a mage. If one knows something the others don't, it should come with a disadvantage. With the Magic system this is not a very big problem, as you have to build your spells from very low scores with your skill points, but if a player wanted to powergame he could just create a spellcaster and choose not to invest any points into his spells. He would be a terrible spellcaster, but yet he would have a slight chance to succeed and eventually he would learn, becoming better and better.
I was thinking of adding a specific profession called "Magic-User". Only Magic-User would be able to cast spells and he could invest his professional skill points to magic and only to magic. Thus he would be able mage and possess some additional skills as well (personality points as well as INT x 10 skill points he could distribute freely), but still he would have severe restrictions when it comes to non-magical knowledge. This would work especially in a setting where magic is considered more or less secret knowledge and learning it would require convicted dedication (I was thinking of Tékumel).
Another system that popped into my mind was having something like OpenQuest, but with two character classes: Fighter and Magic-User. They would have different powers and using them would require spending of power points. Powers could be represented as skills. Magic-User powers would be spells, naturally. Fighter powers would be more down-to-earth in nature. Double damage, additional combat maneuvers, and such. I'm not a great fan of character classes and I don't really like experience points (BRP's learn-as-you-practice makes much more sense), but this could be a solution decent enough. Character classes, yes, but so broad that they just more or less represent characters' general attitude to magic and/or their education and background.
On the downside it would distinguish player characters from regular characters, and I don't know if that is a good thing. Player characters should be the heroes of the story because they are good at what they do, not because they have rules that play in favour of them. On the other hand, one could assume that a cook has plenty of cooking powers--they just don't turn up too often in regular play. What do you think?