One thing (among many) that I love about B/X D&D is the way magic-users (and elves) work. B/X is the only ruleset, out of the myriad of D&D rulesets out there, where magic-users - particularly their spellbooks - works in this specific fashion. A few things that makes magic-users different in B/X:
- Their spellbooks contain a number of spells equal to the number and level of spells the caster may cast in a single day. For example, a 4th level M-U can cast two 1st level spells and two 2nd level spells so their spellbook contains two 1st level and two 2nd level spells.
- There are two ways to acquire new spells: learn them from someone else or spell research.
- Spells may not be copied from scrolls or captured spellbooks into a caster's spellbook.
- Read magic must be used to understand magic writing on items or scrolls so that the scroll may be used later.
The impact of these particularities is that you have a de facto specialist system. Each M-U is different. There may be the famed mystic who lives in a ruined tower on Stormkiller Mountain that can communicate with otherworldly beings (has Contact Higher Plane in their spellbook) or the feared Dark Mage who can raise the dead to obey his command (Animate Dead in his spellbook). Assuming there is only two or three 9th level Magic-Users in a setting, each one can be very different.
Also, it places an importance in finding someone who can teach you the spell that your M-U wants to learn. Your party has on a quest featuring Fire Giants. You think that knowing Wall of Ice might be useful. Who can teach you Wall of Ice? Well maybe the feared Ice Wizard can be talked/bribed into teaching it to you? Instant adventure.
Another important aspect is Spell Research. I don't know about you but I have never seen a M-U research a spell in 1E or 2E. Being the only other way to add a new spell to an M-U spellbook increases the importance of spell research. This is where I do make a house rule that allows a captured spellbook to halve the cost and time of researching a spell that is in the spellbook. I also have no trouble with researching an existing spell that the player doesn't know. For example, say the Ice Wizard cannot be convinced to teach the player Wall of Ice. The player can then decide to research it instead, assuming they have 4,000 gp and 8 weeks of time.
I also allow magic-users to change the spells in their spellbook according to the "Replacing Lost Spellbooks" rules on X11. If the wizard looking for the Wall of Ice spell already had all of the 4th level spells they were allowed in their spellbook, they could opt to replace a 4th level spell that is already in their spellbook. They could then replace, say, Growth of Plants with Wall of Ice once they learned it either from the Ice Wizard or through spell research. This would cost 4,000 gp and 4 weeks in addition to how much/long it took to learn the spell in the first place. This allows some flexibility for magic-users but at a cost.