My personal DON'T list
1. RNG determining the success of a role or item
This lessens the amount of skill, preparation, or planning, and can ruin games for certain people depending on the result. It can be the sole reason a game isn't liked, if used too much or for very important actions. Supps 4 is an amazing example of how RNG can ruin mafia.
2. A Messy and Unnecessarily Large OP.
Now, I get it. Your game has a story that you want to highlight, you have a lot of roles and mechanics that you want to highlight. But there are ways to do this without making your players want to kill themselves. My favorite OP's and the ones that I think are done the best feature the following:
No Scroll Wheels. These condense your OP poorly and make it more difficult to look through. If a player has to scroll through a role list 5 times just to check up on roles mid-game then you have a bad OP. This post should be easy to come back to at any moment during the game to check for a specific detail and scroll wheels do not allow this. An example of how this can hurt an OP is Faeries & Legends.
An incredibly long list of roles with descriptions that lack brevity. If you're going to have a complicated role or several, for that matter, try to explain it/them as clearly as you can with as few words as possible. People are going to say TL;DR and the OP is the LAST thing you want someone to not read as a GM.
Separate CSS boxes for each section of the GM's post. This might just be me, but I feel like having an obvious division between your Role List, Rule list, Lore, etc. in your OP can really make your post not only easier to read, but also easier to find a specific detail for later in the game. Think of this as the header of a website. It makes your OP look cleaner and prettier, which is always good. However, this does increase the length of an OP, so don't use it willy-nilly. You don't want the first post to take the entire first page of a thread. This is probably just my preference, and you can see how much I like it considering I loved using it for Gotham (Thanks Kiyo for the original CSS featuring headers).
3. Taking attention away from the game
Events are fun. They are cute little ways to add some lighthearted fun or some crazy items or actions to spice the game up. They can also be used as devices to implant some fun game lore, there's nothing like a good ol' lore infested event to give your game a plot that the players can get into and follow. Too many events is not fun. The point of your game is to play mafia, and if you have an event every day then your players will lose focus from the game and it often gives lazy players a way to remain active without actually doing anything. The pick-a-present styled event is a commonly picked event. You pick a numbered or colored item, and inside is a neat surprise!
or a cruel and sudden death. I have a love-hate relationship with this event, because it adds some dynamic elements into the game like guns or other items, but this event has its downsides. Players can often just pick their present then not post for the rest of the day, that's no fun! Often after a GM has posted this, you find posts with like 5 words in them, just because all they're doing is picking their present. It takes away from the game. The point of me bringing up this example, is to highlight that the GM should carefully decide what events they should use in a game, and that they should use them sparingly, so that the game can continue as it normally would, but with a couple extra deaths mixed in.
4. RIDDLES
This is the worst mechanic to ever exist. The riddles are often incredibly difficult or vague, attached to an overpowered effect that is meant to use the riddle as a buffer but it DOESN'T WORK because no one signed up for a mafia game to solve riddles! A prime example of terrible riddles and why using them as a buffer is a terrible idea is the Riddler from Battle for Gotham or the Sphinx in Faeries & Legends. It isn't fun, the effects of not solving the riddle is almost never fun, and it always leaves a salty player or two. It's best to either rework the overpowered role by buffering it with something easier to balance or to nerf the role entirely. Riddle events aren't that great either, as they generally take advantage of timezones and once again, no one signed up because they like riddles.
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This is a long post for me and I feel like I have nothing more to say. This probably wasn't what you wanted by asking the question but I really wanted to say this somehow. I considered the OP as someone asking advice on what I personally think GMs do right or wrong, what they can do better, or just what my personal preference on a game is, and I think this post highlights some of that. Mentioning any game or role is meant as an example to solidify my points, and in no way is meant to attack the GM behind that game and it is meant purely as criticism as for what I think can reduce fun in mafia games or make a game less appealing. I hope my wall of text was worth reading.