Old school D&D is my kind of D&D, but there is still a lot of variation in rules, reactions, and assumptions even within the old school. Some people see idiosyncrasies in D&D rules as things which need to be fixed, some see them as features to be celebrated. I guess I'm a mix of both.
Some criticisms of the way magic-users work in older versions of D&D were that they were too weak and that they could only toss a couple of spells and then were pretty much useless. Now I suppose that may be true* if your idea of "useful" means strictly combat, but there is certainly more to old school play than killing monsters.
A magic-user cannot use a sword, we are often told, because he spends most of his time studying with his nose buried deep in musty old tomes. Well doesn't it seem natural, then, that an additional class ability of magic users ought to be obscure and esoteric lore? If there's one thing that 80's television has taught us, it is that "knowing is half the battle!" If the fighting-men have the monopoly on the first half of battle (a strong sword arm), then the magic-users should certainly carry the other half.
When faced with strange runes, strange creatures, or whatever in the dungeon, the magic-user ought to be the go-to guy for legends or rumors or scholarly information about them. A "Lore Roll" should be made, modified by intelligence bonus and perhaps by level, whenever something like this comes up.
For example, if you're using Swords & Wizardry, the single saving throw number could be used for it (that gets better by level, right?) Or perhaps start with a 1 in 6 or 2 in 6 chance (like certain actions in original D&D or the skill system in Raggi's LOTFP:WFRP) and allow the magic-user to gain a pip every few levels.
From the newer versions of D&D, I really like the idea of ritual spell casting. When your magic-user memorizes those precious few spells before adventuring, these aren't the only spells he knows or that he can cast, and he certainly doesn't "forget" them once cast. No, these are spells set up and prepared beforehand to go "pop" when needed. That's why there is only a limited number of slots for them.
A magic user at home in his temple/laboratory/sanctum sanctorum is not so limited and can cast any spell in his spell book as needed. Of course, it takes a long time, must be undisturbed, and probably burns through some of those much hated material components. How much of each? I dunno, let's say an hour per level of spell, and perhaps 100gp in components per level of spell squared (100, 400, 900, etc). Maybe some spells can only be cast this way. I can't imagine Find Familiar as being a spell anyone would prepare for the day when going forth into the dungeon.
What would even be cooler is if regular combat spells could be cast this way. Of course, you would need something to use to target the spell, like some hair or an article of clothing of the victim. Ray of Enfeeblement suddenly goes from a ho-hum dungeon combat spell to a nasty curse placed by an offended wizard upon the head of the town watch.
Magic users aren't useless...they just do things a little differently.
*In most versions (i.e. the couple I looked at) the magic-user has just as much of a chance to hit in combat as a fighter for the first couple of levels, and of course if you're using the "all weapons do d6 damage" rule, a dagger is as deadly as a bastard sword. Useless? Bah!
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