It's always good to have you in these discussions, Samwise. So we also have: Forced fights/resting issues, chassis nerfs to casters, split spell lists & wildshape et al nerfs. Roger.
But holy cow does the "called" spells pre-initiative and no-takebacksies change everything. How far did this go? Could a monster just move to a new square to defeat targeting? Did mundanes have to obey that rule too? Just WOW
The best I can do is quote extended passages.
PHB
"A party of 5 characters - a magic-user, a cleric, a thief, a human fighter,
and a dwarf fighter surprise an illusionist with 20 arcs. The opponents are
30' distant, and the magic-user immediately begins casting a sleep spell.
The cleric also prepares to cast a spell, silence, 15'radius. Meanwhile, the
thief darts to the rear of the party to attempt to hide in the shadows and
attack from behind when opportunity presents itself; the human fighter
nocks an arrow and shoots it at the illusionist; and the dwarf hurls an axe.
The surprise segment is over, and initiative is determined. The
illusionist/orcs win initiative, and while the former begins a spell of his
own, the latter rush to attack, hurling spears as they come. A spear hits the
magic-user, so the sleep spell is spoiled. The orcs are attacked by the
fighters, the cleric casts his spell upon the illusionist, and the magical
silence both spoils his prismatic spray spell and enhances the chances for
the thief's attack, for he is successfully slinking and sliding around in the
shadows. Thus, after surprise and 1 melee round, the party has inflicted 2
hits upon the illusionist, spoiled his spell attack, and felled one orc and
wounded another. They have taken 3 spear hits and had one of their spell
attacks ruined."
We can see:
The party has surprise.
A 1st level spell with a casting time of 1 segment is NOT completed during the surprise segment.
The minimum 14th level illusionist elects to use his 7th level spell in response.
That gets ruined by the 2nd level spell. (Yes, that trick is THAT old.
)
DMG
(This section is extensive, so I'll just go with some highlights.)
The Set Up:
"Spell-casters will always insist that they are able to use their powers during
combat melee. The DM must adjudicate the success of such use."
The Complication:
"Thus, casting a spell requires that a figure be relatively motionless and concentrating on the effort during the entire course of uninterrupted casting. For example, a magic-user casting a fireball must be in sight of the intended area of effect during the course of the spell (although an associate could be there to open a door intervening between caster and target area at an appropriate time - provided the timing was correct, of course). The caster cannot begin a spell, interrupt it just prior to completion, run to a different area, and then complete the spell; interruption instantly cancels it. Unless a spell has no somatic components, the caster cannot be crouching, let alone prone, during casting."
The Gimmick:
"It can thus be understood that spell casting during a melee can be a tricky
business, for a mere shove at any time can spoil the dweomer!"
The Rules:
"1. Spell casters must note what spell they intend to cast at the beginning
of each round prior to any knowledge of which side has initiative."
"3. Intelligent monsters able to recognize the danger of spells will direct
attacks against spell casters if not engaged by other opponents so as to
be prevented from so doing."
The Kicker:
"Because spell casting will be so difficult, most magic-users and clerics will
opt to use magical devices whenever possible in melee, if they are wise."
Now it does not absolutely say "no casting in combat", but it is pretty clear that RAI is RAW, "don't let your players cast in combat!"
To some extent precise targets were a tad fungible - you could probably shift your sleep spell 10' over, or go for the wizard instead of the cleric with your magic missile, but if you decided two orcs weren't worth a fireball then too bad, you already started casting it, fry something or lose it. (Oh, and have I mentioned physics and fireball blowback? MUOHAHAHAHAHAHAHAHA!!!)
Oh, and don't forget facing. Read the above and consider whether turning completely around to scorch the flanking reinforcements "ruins" a spell.
And to a certain extent, yes, mundanes could be trapped by their declared actions. "I attack this guy here" means attacking in that particular square, not running 30' and attacking someone completely different on the other side of a door, or suddenly deciding that using a potion of extra-healing was more relevant. Attacking Orc 2 instead of Orc 1 if they had been standing next to each other was something I allowed, and I suspect most DMs allowed, but that was about it.
Remember, D&D grew out of miniatures wargaming and originally had 1 minute turns.
You wrote down your orders, handed them to the referee, and he resolved your "simultaneous" actions as best as possible. If a unit broke and fled, then your artillery barrage hit an empty field, and that's all there was to it.
That changed a bit with the move to 6 second rounds (previously segments) in 2nd ed, and only completely altered with individual phases rather than team initiative in d20.
