The core rules for this come into play regarding bonus action spells only, however, they have some fairly ridiculous repercussions:
A high level fighter-wizard can: Cast meteor Swarm, have an enemy attempt to counterspell them, counterspell that enemy's coutnerspell, then action surge and cast an 8th level disintegrate on the upstart enemy caster. That's perfectly legal inthe core rules.
That exact same wizard might also attempt to first cast Shillelagh on their staff... the enemy caster counterspells this because they were waiting to counter regardless. The wizard then CANNOT counterspell that counterspell -that would be casting a levelled spell on the same turn as casting a bonus action spell. The also cannot cast anything with their action other than a cantrip, and even further, even if they then action surge, they still cannot cast anything other than a second cantrip with their extra action, becuase it's still their turn and they still cannot cast a leveled spell, because of the bonus action.
This is made even more ridiculous by the fact that the rule doesn't need to exist - the vast majority of bonus actions spells are paladin and ranger spells, made to accompany using your action to attack, not cast more thing. They're all concentration in those cases too. If we remove all of the arrow, mark and smite-type BA spells (and others that rely on using your action to do something other than casting), we're left with a meager handful of spells indeed: barely a dozen of them in fact, the most notable of which are: Far/Misty step, Healing/Mass Healing word, Expeditions Retreat, Sanctuary and Dragon's Breath.
There's no real break or abuse in any of this.
Considering that a Paladin/Fighter of the same level as our Wizard above can nova off literally their entire daily allotment of spell slots in a single turn (over 11 spell slots), and all as guaranteed damage, perfectly legally, it's a bit strange.
Where it DOES become dangerous and abusuable is, as others have mentioned, with Sorcerers and quicken... and even then it still doesn't really measure up to the above paladin/fighter, and it doesn't do anything more or worse than the two-drop-levels-of-figter wizard action surging to cast multiple fireballs, etc. Same exact effect but perfectly legal... but it is still a potential for a lot of AoE, and other combinations that can be clever and strong (as though that's bad..?). This is the only place where the real breaks show up though... which means that the rule about bonus actions SHOULD be a rule that is written into Quicken Spell specifically, and leaves casters everywhere else alone.
(In games I play in, in one of which I play a sorcerer, we discard the bonus actions spell rule entirely, and instead rewrite quicken spell meta magic to read: "When you cast a spell that has a casting time of 1 action, you can spend 2 sorcery points to change the casting time to 1 bonus action for this casting. If you do this, you can't use your action to cast another spell on the same turn, except for a cantrip." It's simple, it's straight forward, it fixes the exploit they wanted it to without impacting anyone else, and it works.)
I'm personally GLAD they've ditched that rule, and it's the one place where I absolutely support them diverging from 5e, because the arbitrariness of that bonus action casting rule simply does not need to exist, and is a legacy left over from impressions of casters in earlier editions which no longer hold true in 5e.
Very good points !
Originally Posted by DiDiDi
I wish both 5e and Larian adopted the fixed version of the rule:
A character can only cast one non-cantrip spell during their turn, no matter what actions they have available to them. They can combine that spell with any number of cantrips that they have the actions for. This does not restrict spells cast not during your turn, such as a Hellish Rebuke cast as a reaction.
If I still played P&P, I would adopt this rule as a DM. Wizards/Sorcs are OP even without completely ditching that rule...
Would be more balanced or safer. Niara's examples does show odd discrepancies (in tabletop 5e), where the Action Surge combo is oddly better than a pure caster :P.