Haste already says you can’t use the action it grants to cast a spell. If quickened spell had a similar thing (“If you quicken a spell you can’t casts another leveled spell on the same turn.” or something) it definitely wouldn’t be an issue.

Ah, yes, sorry. I mixed up my terminology a bit there, good catch. Every instance of “round” in my comment was supposed to be “turn”. I’ll edit it. But yes, sneak attack is also once per turn, and not round, which is very odd. It honestly seems like an oversight that just happily caused the balance for the rogue to catch up a bit. Rogue doesn’t really have any ways to consistently trigger it, and while it seems like it might be a case of “extra attacks should get the same effects as regular attacks” (if that makes sense to you) then it is extremely odd that the Barbarian’s advantage from Reckless Attack doesn’t last for the round, only for your own turn. So AoOs don’t have the advantage.

Honestly, as far as I’ve seen most spells aren’t an issue. Only sorcerer quickened spell really makes it an issue, but that’s mainly an issue with quickened spell rather than anything else.

I also believe Jeremy Crawford or someone has mentioned that balance wasn’t the concern when the role was put in place. I’m not able to look for the source right now, but I think Treantmonk had it in a video about this rule.

Yeah, the more I play DnD and other games, the less I end up liking 5e’s system of action, bonus action, reaction. Systems that just have actions are much more appealing, imo.

I don’t like this rule at all. Definitely among one of my least favorite rules in 5e. There are several things wrong with this rule. First, the stated reason why this rule exists is not balance, but it exists to make sure that a spellcasters turn isn’t taking too long, by limiting them to only one ‘noodly’ spell per turn to stop them from flipping through the books trying to find the two perfect spells per turn, rather than just one (cantrips are easier to remember and use, I suppose). Unfortunately it fails at this in my opinion because of reason number two: the placement in the book. The rule is listed under the “bonus action spells” header in the spellcasting section. This is right between the “action spells” and “reaction spells” sections, and both of those just say “You can casts a spell with an action/reaction” and have no real rules. So people basically glance over it and assume there’s nothing important there. This means that new players (thepeople who will take a ton of time on their turns if they have to find two spells) don’t know this rule exist. The people who do know about this rule don’t need it, because they already know what spells they want to use and are much faster at taking their turns (hopefully). Also, the fluff is entierly nonsensical “Because bonus actions spells are espescially swift, you [can’t cast other spells on the same turn]”, what? Wouldn’t it make more sense that swift spells would leave you with more time to cast other spells?

Honestly, it’s even worse than that, because once you know the rule it actually causes the game to slow down because of how noodly it is. When you are casting a spell you stop and think “Wait a minute, is this allowed according to the bonus action casting rule?”, and then you have to find that out (hopefully not on your turn, but it causes you to have to look up this rule more that you really should have to look up any rule). If I am DMing I really don’t care about my players following this rule, but if I am playing I will always follow it to the letter (unless the DM says otherwise, of course), because I have had to look this rule up so many times I can now quote it verbatim from memory.

I think that this rule could be ok, but it needs some changes. First it needs to have it’s own section in the rules book “Casting multiple spells in a turn”, or something. Don’t hide it among stuff people skip over. Second, it should probably be changed to just “Because the casting of spells is a taxing affair, you can not cast more than one spell of first level or higher in the same turn.”. This is how most people think the rule works anyway, the fluff makes a ton more sense, it is simple enough that you don’t have to look it up constantly, and as a bonus it finally would answer all those people who are very confused about how you can cast counterspell in the middle of casting your other spell (you wouldn’t be able to, because that would be two leveled spells in a turn, except it you are counter-counterspelling to save your cantrip, I suppose. But that’s a very strange edge case.)

As a testament to how terrible this rule is, not even this is the right one. The rule is, when you cast a spell (including cantrips) with your bonus action you can’t cast any other spells except a cantrip with a casting time of one action on the same turn. So casting Shillelagh stops you from casting leveled spells and (although I’m not sure why you would want to) from using your action to start or continue casting a cantrip like Mending, because it has a cast time of 1 minute (Aka 10 actions, aka not one action).