In AD&D (1st edition), spellcasters still destroyed everything at the highest levels, particularly the wizards. Before then, they had a few more issues.
How many of these would slow them down in D20?
Which would be "acceptable"?
Wizards and Illusionists were hit with:
1. Spells Known Per Level
Chance to Minimum Maximum
Ability Know Each Number of Number of
Score Listed Spell Spells/Level Spells/Level
9 35% 4 6
10-12 45% 5 7
13-14 55% 6 9
15-16 65% 7 11
17 75% 8 14
18 85% 9 18
19 (25) 95% 10 All
20 (27) 96% 11 All
21 (29) 97% 12 All
22 (31) 98% 13 All
23 (35) 99% 13 All
24 (39) 100% 16 All
25 (43) 100% 17 All
Unlike a Spellcraft check, which can optimized to be guaranteed, there was pretty much always a failure chance at "mortal" levels of Intelligence. And of course unlike D20, there were vanishingly few ways to get an ability score over 18 barring wishes.
Even with the other boosts available in D20, the numbers in parentheses for ability scores reflects treating them as increasing the way giant strength did between the editions. (Hills giants had 19 Str in AD&D, Titans had 25 Str. They have 25 and 43 respectively in D20.) While still doable, it is a bit harder these days.
The last column leads into the next major hurdle all spellcasters faced:
There weren't that many options.
2. Spells Available Per Level
Cleric Druid Magic User Illusionist
Cantrip 0/0 0/0 0/66 0/74
1st 12/20 12/16 30/40 12/16
2nd 12/20 12/16 24/36 12/16
3rd 12/20 12/16 24/32 12/16
4th 10/16 10/12 24/32 8/12
5th 10/16 10/12 24/30 8/12
6th 10/12 10/12 24/30 8/12
7th 10/12 10/12 16/24 5(30)/7(40)
8th null/null null/null 16/20 null/null
9th null/null null/null 12/16 null/null
That's how many spells were in the books - PHB/Unearthed Arcana. (The number in parentheses for illusionists at 7th level indicates they can learn 1st level wizard spells as 7th level illusionist spells.)
So even when a wizard might know all spells of a level, there weren't as many choices available.
Clerics, druids, and specialist wizards in particular have extremely cut-down spell lists.
There were some more expansions in 2nd edition, which led someone to decide to try and control clerics by introducing the concept of "spheres" - which were sort of glorified domains - to restrict spell access. The "best" part of that being not allowing the "healing sphere" to specialist clerics of evil deities.
Then there were two power source specific controllers.
Clerics and Druids got:
3. Code of Conduct
Not explicit like the paladin code, but:
Cleric spells of third, fourth, and fifth level are obtained through the aid of supernatural servants of the cleric's deity. That is, through meditation and prayer, the cleric's needs are understood and the proper spells are given to him or her by the minions of the deity.
Cleric spells of sixth and seventh level are granted by direct communication from the deity itself. There is no intermediary in this case, and the cleric has a direct channel to the deity, from whom he or she receives the special power to cast the given spells of these levels.
. . .
If they have not been faithful to their teachings, followed the aims of their deity, contributed freely to the cause and otherwise acted according to the tenets of their faith, it becomes unlikely that they will receive intermediary aid unless they make proper atonement and sacrifice. There can be no question that such clerics must be absolutely exemplary in their activities, expressions, and attitudes if they dare to contact their deity directly!
Still the whole "Play the way the DM wants you to play or get totally hosed."
Wizards and Illusionists got hit with:
4. The Spellbook
In many ways, all the same things are said in D20, but in AD&D, the spellbook was considered a primary point of vulnerability the way a familiar is in D20.
Ordinary spellbooks are big:
The encumbrance value of such a book is equal to three times its weight (450 gp or thereabouts), although it is correct to assume that a volume will fit within an otherwise empty backpack or large sack.
Even a travelling spellbook is hefty:
Five such books will fit within a backpack, twice that number in a large sack.
And neither held that much:
1. Standard books, each of which contains up to 36 cantrips, up to 24 spells of 1 st-3rd level, up to 16 spells of 4th-6th level, or up to 8 spells of 7th-9th level.
2. Travelling books, each of which contains at most one-fourth of the number of spells possible to be contained in a standard spellbook - either nine cantrips; six spells of lst, 2nd, andlor 3rd level; four spells of 4th, 5th, andlor 6th level; or two spells of 7th, 8th, and/or 9th level.
And implicit in the concept of travelling spellbooks is that they are a point at which to doubly hit wizards where it hurts - not only do you menace their source of spells, but you force them to carry only a subset around with them.
Back in AD&D days, the Spell Mastery feat would have meant something.
And the primary D20 method of optimizing a spellbook - (Boccob's) Blessed Book, worked much differently in AD&D. It was simply smaller (12"x6"x1"), gained a +3 save bonus, and could contain 45 levels of spells.
Not at 1 page per spell, just 45 levels of spells.
And not at a reduced cost to scribe either. Does anyone count spellbooks against WBL for wizards?
As another major hit, druids simply didn't have the Big 2 - wild shape and animal companion:
5. Druid wild shape was essentially the Ranger ACF version - tiny to medium (bullfrog to black bear), get animal ability scores, attacks, and movement methods, heal some hit points, and . . . that's it. 3/day.
No large or huge; no plants; no elementals.
Animal companions were the 3E version rather than the 3.5 version - use the animal friendship
spell for 2 HD/druid level or ordinary critters with no enhancements. If you were lucky enough to prepare the spell when you met a tiger, you could befriend it, otherwise be happy with your common woodland creatures. And remember, no "dire" animals or what not back then either.
Finally, there was the issue of:
6. Spell Memorization time:
15 minutes/spell level
In D20, a 9th level wizard knows 34 levels of spells without Int modifier or specialization.
That is 8-1/2 hours to replace his spell list in addition to the 8 hours of rest.
While it is "default" that high level wizards just wander off to alternate time flow planes, at more "modest" levels, the standard lurking in extra-dimensional spaces becomes a bit less optimal. The "4-hour workday" swiftly turns into the "4-hour workweek", completely trashing any concept of the non-static dungeon if parties let their spellcasters blow through their available spells then wait for a full reload.