Meta gaming ruins the game, makes it less fun, and takes away most of the skill aspect. Things like reading off your role's description make the game unskilled and removes the social deduction aspect of the game.
The order of player names can get shifted around as they wake up, and it can give players an unfair overview of who is performing actions at night. Talking at night gives an unfair advantage to the Villagers, as info-based roles such as the Clairvoyant can call out who they checked, even if they would have otherwise died that night.
Typing during the Mayor Election, the Lynching, and at Night is not allowed, to prevent misclicks. Players make choices on who to perform actions on via text in the chat, and typing moves this text up, potentially making players choose the wrong person.
This rule is in place to prevent players from having an advantage over others, using an external program - Werewolves with Optifine zoom enabled can see the Little Girl easier, and the Little Girl with Optifine zoom enabled can see the Werewolves' names from further away.
Players who leave mid-game will not have their win rates modified, regardless of being in Competitive or not. Furthermore, players who leave mid-game and rejoin later can cause bugs by having leftover information from their previous game still attached to them. Leaving mid-game to save your win rate from dropping is toxic and should not be tolerated.
Private messaging unfairly benefits the Villagers by allowing players to secretly strategize together. The only allowed PMing is teammsg, which can be used by the Werewolves at night via /tm or /teammsg in order to discuss who to kill and what to do the following morning.
Using commands like /kill, /gamemode creative, besides being unnecessary and often annoying, can easily break the game if used at the wrong time.
Pretty self explanatory - don't waste peoples time in votes during the day, it only chews through time and annoys everybody.