I was watching a video on YouTube where a guy was evaluating the 5E classes from worst to best, as he saw them. I may watch it again and write down some comments/criticisms of it, as he has some rather vague criteria and his reasoning for why certain classes are good is also cited as a reason why at least one class is bad in his opinion. I won't link the video yet. If I do critique it, of course I'll link it then.
As I was watching it, I was trying to figure out how he was rating each class. And really, it boils down to three things: how versatile is the class, how "cool" did he find it to role play, and how well did it take advantage of the "action economy."
The concept of the action economy is a relatively new one in D&D, but it's been around in games like the Palladium system for a long time. If you pretty much stick to OSR blogs/forums, you may not be familiar with the term. The action economy is the idea that characters can take X actions in a turn, and if they don't take full advantage of these actions each round in combat, they are letting the side down by being inefficient.
In 5E, on your turn each round you can move your speed, perform one "action" and possibly perform one "bonus action." I think you can get one "free" object interaction as part of the move and/or attack, like drawing a weapon or opening a door. And during other players' or the monsters' turns, you can get one "reaction" per round.
5E of course has a predefined list of possible "actions" one can take. And yes, scare quotes because you can't just do any old action you can think of. Well, you can, but whatever it is it will fit into one of the predefined categories of action in the book. That list is: Attack, Cast a Spell, Dash, Disengage, Dodge, Help, Hide, Ready, Search, Use an Object. (Higher level warrior types get an ability called Extra Attack which lets them make more than one attack when they take the Attack action but it counts as just one "action" for the action economy.)
Bonus actions are not to my knowledge ever listed out precisely, because they are basically exceptions to the normal rule. If a class ability, racial ability, or spell grants you a bonus action you can take it. Otherwise, you get no bonus action. Monks, for example, can always make an unarmed strike as a bonus action IF they take the Attack action. At 2nd level, Rogues can always choose to use a bonus action to Hide, Disengage, or Dash. Clerics who cast Spiritual Weapon use a bonus action to make the hammer attack.
Reactions take the place of "attacks of opportunity" in 3E. And they aren't always just attacks, although some are. Many are class abilities that let you avoid or reduce damage (Monks can deflect arrows, Rogues can reduce damage from one attack, Wizards and Sorcerers can cast the shield spell).
In good old D&D/AD&D, you don't have to worry about all this. Everyone can move and do one thing on their turn. Much simpler. And in a fight, not every character in the party was expected to do something each round.
The action economy does add a layer of tactical complexity that many people enjoy. I understand its appeal. The problem is that the action economy seems to be one of the main considerations people like the guy who made the video I referenced above have for rating both the classes of 5E but also how people play the game.
If you don't take full advantage of the action economy, if you don't take a class that makes use of bonus actions and reactions, if you squander your turn, it's seen as letting down the side. Not pulling your weight. Being lame and useless. Totally sucking at the game.
It all comes back around to the fault WotC had when they created the game. It's all about combat.
The by the book primary source of XP awards are for combat. Most character abilities are designed to help you in combat. Most spells are designed to help you in combat. The action economy is designed to help you optimize combat.
Computer RPGs are all about combat because it's still difficult to program into a game the sort of freedom you get with a tabletop RPG. Why WotC decided to limit their design of 5E to mimicking a computer RPG is beyond me. I mean, shouldn't they have learned their lesson from doing that with 4E?
OK, I feel like I"m starting to ramble. It's getting late. Let me wrap this up.
The concept of an action economy is fine in and of itself. It does add a level of tactical variety to the game, which many people like. And yes, it can be fun to take advantage of it. But what started as a tool to add variety and fun has become a yardstick or straight jacket on the game. Too many people are looking at and evaluating game mechanics and more importantly game play based primarily on how well a class or build takes advantage of the action economy. Bonus actions are not seen as a bonus, they're seen as a necessity. And the attitude I'm seeing more and more is that if your character isn't taking advantage of bonus actions and reactions as often as possible, you did something wrong or are playing wrong.
It's valuing system mastery over immersion and creativity, prioritizing optimal combat efficiency over playing your character. That's why it's bad for the game.