Nor was it the only one. There were
numerous other restrictions on spellcasters that were later jettisoned:
- Wizards had to make a check to learn every new spell they came across, which after 1st level was the only way to gain new spells (i.e. you didn't gain new spells automatically as you gained levels). If you failed that check, you had to wait an entire level to be able to try and learn that spell again.
- Clerics didn't actually get to pick their own spells. While low-level spells were gained through faith alone, higher-level spells had to be requested from your deity (or their divine servants), who might deny your request in favor of granting you different spells. So if you asked for an earthquake spell, and the DM knew you'd be facing a lot of undead, he might give you a spell that was more in tune with what you'd need instead.
- Wizards could only learn so many spells of each spell level. I don't mean memorize only so many, either; they had a maximum number of spells (based on their Intelligence) at each spell level that they could learn. So they had to judge each new spell they came across carefully to consider if they wanted to add it to their limited repertoire.
- All spellcasters automatically failed what we now call "concentration checks." If you were distracted, let alone injured, while casting - which was a very real possibility, as spellcasting times were measured in "round segments" that quite often overlapped other character's actions - you automatically lost the spell, no check.
- Magic item crafting was something spellcasters could do automatically at certain levels, but was never just a matter of time, gold, or other predefined resources. It always required esoteric components, which were determined by the DM. If you wanted to make a wand of fireball, you conducted research (or paid a sage to do it for you) in order for the DM to eventually tell you that it required phoenix feathers, the blood of a noble efreeti, and a fire ruby that had been dipped into an active volcano. And just like that, your wizard now had three new adventure hooks that they were pressing the rest of the party to pursue.
- The target numbers for saving throws were entirely determined by the characters/creatures you were attacking; they weren't set on your end. So if a high-level fighter had a "saving throw vs. spells" of 3, they just needed to roll a 3 or higher on their d20 roll against incoming spells, which meant that they were going to make their save almost all of the time. The best you got was to maybe inflict minor penalties to their roll if, for example, you were a specialist casting a spell from your favored school of magic.
- Magic resistance worked differently depending on your edition of choice. While in AD&D 1E your chance to overcome MR varied depending on your level (specifically, the listed magic resistance for a creature presumed that you were 11th level; for every level below that, you added 5% to the value, and for every level above, you subtracted 5%). Conversely, in AD&D 2E the percentage chance of magic resistance was absolute. So if a monster had MR 50%, then half of your spells were going to fail regardless of whether you were a 1st-level apprentice or Elminster himself.
- There were fewer safeguards written into the spells themselves. If you went off-target with a teleport spell, and ended up inside a stone wall, you were dead. No check, no saving throw, no "you shift to the nearest open space and take X damage." Everytime you used polymorph self, you made a system shock check, and while the percentage of success was slanted in your favor, if you failed then you died instantly. (On a related note, resurrection spells also had a chance of failure, and while that one was even more slanted toward success, if you failed it you were permanently dead barring divine intervention. Of course, you could only be resurrected as many times as your Constitution score anyway.)
- The level of spells that clerics could gain was partially dependent on the power of their god. Demigods could only grant up to 5th-level spells, and lesser deities could only grant up to 6th-level. This was supposed to be counterbalanced by higher-level clerics being big fish in smaller ponds, meaning that you had an outsized role in your deity's religious organization (potentially among their divine servitors as well as their terrestrial church), but that message seemed to be lost quite often.
Those are just off the top of my head, but these were how AD&D avoided "linear fighters, quadratic wizards." But a lot of people didn't like restrictions on what they felt should have been super-powerful high-level spellcasters, and summarily ignored some or all of these. When they made D&D Third Edition, Wizards of the Coast listened to those complaints and did away with all of these restrictions, only for the Law of Unintended Consequences to come back and bite them hard, as 3E came to be known as "caster edition."