Strictly enforcing alignment. I had a DM who constantly harped on alignment. "Your character is Chaotic Good, do you think they would really do that?" "Oh, you want to help that person? Doesn't sound like the action of a True Neutral character to me." "No, you can't do something like that. You're playing a Lawful Good paladin." Not all said to me, but to various players.
Yeah, I noped out of that game quick and basically made alignment meaningless in the next game I ran.
Yeah, how alignment is used needs to be covered in session zero. I have two modes for alignment in my games.
1. Alignment as Flavor
This is the most common. Alignment is just an indication of the player's aspirations for their character. There is no real mechanical use of it in the game. Their actions will have repercussions in terms of their reputations, making enemies, etc., but I don't care what they write down as alignment. If they find an alignment-limited magic item I either (1) ignore it, or (2) have an intelligent magic item make a "decision" based on whether it feels the character is "good" or "lawful" or "evil" based on what it learns of the character's past actions, which it can access as part of attunement.
Some players will not like this because if their sheet says Lawful Good, they don't want the DM to say that the Uber Sword of Awesomeness has judged the character not sufficiently lawful.
2. Alignment as a Core Mechanic
In this kind of game, the gods are real and existence is divided among the alignment scale. You are expected to take a side and be held to it. Clerics who don't act according to their alignment will find powers denied to them and atonement needed--or to find a new god. Warlocks may find that their Patron doesn't care what they feel in their hearts, but will expect the character to act in accordance to the Patrons alignment. The ability to attune to more powerful magic items will often be tied to alignment. Certain creatures will respond to characters differently depending on their alignment. Some wards and rune will only trigger for creatures of certain alignments.
When I run style #2, I never say "your character will never do that". Instead, I keep a track of violations of alignment and the player will eventually start to notice subtle changes in the potency of the magic, they may lose attunement with a magic item, they may be turned away from certain temples, etc.
Because this is something that depends heavily on rulings over rules, I allow and encourage the players to have their characters visit temples and give confession and receive judgment and instructions on how to make amends.
But because 2 is more work and give me too much influence on how players play their characters, I generally avoid it unless it is a heavily religious-themed campaign.
