3.0 was a mess, but 3.5 still had a lot of guardrails that 5e removed or disabled with hamfisted attemptsto compensate in other ways that cause significant problems Here are some examples
- Concentration :: d20srd.org
- There used to be a lot of things that you could do in combat to provoke an AoO, one of them being...
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- doing that would trigger a Concentration check of 10+damage taken where failure results in loss of action, loss of spell slot you were hoping to use, & failure to produce any sort of effect.. The 5e dc is now 10 & only climbs to 11 when they get hit for 22 points of damage with every two points of damage raising the dc by one even though the concern is likely more "that attack took like half my health!" so the dc is fairly trivial to meet
- Many of those other things that provoked an AoO allowed martials to influence control over the enemies by making the area around themselves a scary place to be doing things that PCs geerally don't want them doing.
- Casters always had level zero spells, but they were very much not at-will like cantrips & tended to be so bad that even badly using a crosbow or sling was often more desirable.
- Giving casters at will cantrips took that magic crossbow/sling/etc out of the list of things a caster might potentially be interested in & largely removes them from caring about treasure found by the party because they know finding wands & such is unlikely
- Because casters have at will cantrips their see saw of poor to earthshattering damage was stabilized & no longer felt very special or even capable of reversing a tpk in motion when they fully unloaded because slightly below average to slightly above average if there are no resistances is pretty bland & uninspiring.
- Casters could A crank party effectiveness up & enemy effectiveness down to a significant degree & Bwhen a caster specialized in that did their equivalent of "going nova" it could completely defang an encounter while allowing the rest of the party to stomp all over the baddies.
- A was handled by directly nerfing the important elements of size/duration/effect on a by case basis for individual spells & frequently given a middle finger cherry on top by raising one of the meaningless elements like size/duration/range. B was dealt with by saying "well lets just make it so all of those spells are concentration"... but concentration checks are almost guaranteed now so they changed it so you could only concentrate on one concentration spell at a time. Bizarrely A & B were dealt with individually in isolation in many cases while ignoring that the two provided a multiplicative nerf that makes that entire style of casting feel pretty bland & uninspiring to a degree even worse than nuking to the point where a caster specialized in this type of casting was no longer capable of even pretending they meaningfully shifted the dials or were capable of halting a tpk in progress.
- in 3.5 resistance & damage reduction were basically the same as each other where any damage but x damage or any damage that was x damage got reduced by a set amount (2/5/7/10/20?) hurt melee more than casters because melee needed to switch weapons to bypass it & each attack was chipped away at by that 2/5/7/etc while a blasting caster could just cast some other spell if they had one... Again they did two things in isolation without really considering how they combine over time.
- C they decided to equalize the impact by changing it from set values to half damage but didn't consider that the martial usually has multiple smaller attacks that each apply a stat mod as well as any weapon modifiers while the caster generally has one big one with no modifiers & only applies them once even if they do have modifiers.
- D they removed the nuance that made some monsters more difficult for certain types of weapons that were likely to be in the hands of certain classes and just said "to nonmagical bludgeoning piercing & slashing" which had the secondary effect of making it little more than a waste of ink & page space if it was a monster not likely to be something a low level party not yet sporting magic weapons was capable of fighting. Not only that, they did this without considering A & B all that much if at all
- They normalized damage earlier in A & B to prevent casters from ever trivializing an encounter by going nova as described earlier. Combine that with C&D and you have a situation where casters go from merely average but rather uninspiring to being little more than a waste of space
- This is just the tip of the iceberg, I've not even mentioned things like asf, aster level checks, SR, & so on
