So you want to play a Magic-user in 1st Edition? Let’s talk about what to expect, how to use it, and how to make it your own (besides personality, of course).
Magic-users are limited to humans, elves, and half-elves, with Illusionists also available to gnomes if you can roll it. Non-humans are limited in their level, half-elves 6-8 and elves 9-11, depending on intelligence. These limits are below the point at which magic-users make strongholds (level 12), hampering their potential for post-adventuring play, but they are capable of multiclassing as fighters, thieves, or even clerics, which can diversify and extend their adventuring career.
To be a Magic-user, the character must have at least 9 Intelligence and 6 Dexterity (of course, this limit applies to all classes but clerics, but isn’t mentioned anywhere else), but 16 Intelligence or more buys you a 10% XP bonus. Almost every point of intelligence above 9 you can muster matters for you magic-user, granting the possibility to use higher levels spells (a 10 is needed for 5th level spells, 12 for 6th, 14 for 7th, 16 for 8th, and 18 for 9th) as well as improved chances to know more spells in each level group (10, 13, 15, 17, 18 and 19 are all thresholds for improvement). In 1e, raising abilities is very much possible through magic, so these breaks can be met later in the character’s life.
Other abilities carry the same benefits they have in general for all classes, but Intelligence is really the vital one for the magic-user.
Magic-users get a wide variety of spells, although at low level their known spells and their castable spells are pretty limited. What spells your magic-user knows at the start are randomly determined, as is their ability to learn each spell, so it’s important to be able to make do with what you do have, which will always include some good utility. In combat, you’re going to want to stoop to using a weapon from time to time.They get decent ranged weapon options in the dart and the dagger, so at least at low levels you can keep up with the rest of the party for damage as long as you’re loaded up.
Scrolls, wands, and staves are invaluable for shoring up the magic-user’s access to magic, and magic-users get a wider variety than most to choose from, if you can find them. At higher level (11th), magic-users can create scrolls and enchant their own items given enough time.
Magic-user customization is in some ways automatic. The DMG allows a 1st level magic-user but read magic and three other spells bestowed by his teacher when first setting out. These are determined at random, so which of the 30 spells is in your spellbook will greatly influence how the magic-user is played. As your magic-user acquires more spell options, this ability to differentiate yourself still remains, whether you use your spell slots in combat, to avoid combat, to solve problems outside of combat, to manipulate people, there are lots of ways your character can be molded through spell use.
Perhaps the ultimate option of making your magic-user different from any other is spell research and invention. Regardless of your character’s level, it is possible (unless you’re full up on spells for your intelligence) to do the labor of inventing a spell. This is expensive and you will need to be careful that the spell you are attempting to invent is reasonable, because all of the cost can be wasted if you spend for a 1st level spell but have proposed what should be a 2nd level spell. What makes a character more unique than having a spell in his book named after himself? (And the DM is counselled not to make this spell available to any other character.)
This is something any magic-user will probably want to help survive to high levels, but henchmen of various forms both to adventure with and to help with alchemical and other research can have wildly varying personalities that help to make playing that magic-user feel different through the people he surrounds himself with.
Finally, if your DM is using the Wilderness Survival Guide and/or the Dungeoneer’s Survival Guide, then like all classes your Magic-user can present a wide variety of expertise in survival proficiencies. Magic-users start with as many non-weapon proficiencies as anyone else and gain them at a good rate, although unusually spaced 2 every 6 levels (this was clearly to simplify the tables, I’d give 1 every 3 as a matter of policy).