The Crimson Binome
Hero
I'm not sure about the barbarian deciding whether to power attack or not, because combat instinct is hard to understand when you're not there, but those other things are examples of poor role-playing that are likely to be called out as such. Not all players ignore their dump stats, and not all players pretend that their characters know what resistances a monster has (if the player even knows them).I get your point, but PC meta-game far more then DM's, and its not close. Every PC guide here has dump stats, players follow those and then act like it didn't exist. They have Barbarians in rage using GWF feat making the decision not to use that feat when advantage is cancelled out for one round then using it again, and every veteran player "tells" their PC's exactly what resistances monsters have that the PC has never seen before. Stuff like that all the time.
The DM has to put in a lot more work in determining what their characters do and do-not know, but it's still the same basic process as for the players. When determining whether an ogre is wearing plate armor, that determination can only depend on in-game factors (such as whether that ogre is working for a fire giant), because basing that determination on external factors (such as whether you want to challenge the player at the table) ruins the integrity of the model; there's zero satisfaction for a player to reach a goal post that was intentionally moved so that they could (or could-not) reach it.
The DM is also working without oversight, which means the players need to trust the DM to play fairly. A bad DM, who abuses that trust by meta-gaming, can put players off from the hobby entirely. From what I understand, this was a major issue in the eighties and early nineties, with players being paranoid that the DM was out to get them - because earlier DMs, in true adversarial style, were out to get them.