I am a fan of letting players know up front what sort of campaign they will be experiencing, both in-game and at the table as friends. I find it useful to have a set of guidelines for the table, so they know what to expect in terms of both play and behavior while playing. This is by no means an exhaustive list of what I like to tell players up front (depending on the campaign I run, of course - each campaign may be different with regards to table rules), but when running older games meant to be played a certain way (BECMI D&D, for instance, my favorite iteration of the game, or S&W), these are the guidelines I put forth. I plan on having them out on the table, or on a whiteboard, or on the outside of my GM screen during session 0 and session 1.
Here we go:
* Life is cheap in old style games sometimes, and so are character sheets. Have 3-5 additional characters ready to go, since this may be a high-mortality campaign. You may level up each of your other characters that aren't being played at the same rate your current character is earning experience points, so they may be called upon in the event of death. Plus, creating characters is fun. No switching, however, until someone dies, unless you can make a case for it.
* That all being said, I won't kill your character. You will. Running is not cowardly, it's smart. If you venture into an area to nasty for your current characters, retreat is expected and intelligent. Likewise, don't stick your head or hand in a hole before checking it out thoroughly. You never know what's in there.
* If you steal from another party member, or attack them, I wouldn't be surprised if you disappeared in the depths of the next dungeon crawl. I will look at any motivations for doing this kind of thing, meaning starting intra-party conflict. If it's player vs. player motivated, I'll take action to remove one of you from the game permanently. If it just makes sense in the game that the character acted this way, I want everyone to realize that, and respect it. It's a game of choices, and sometimes choices like this make sense in the story, even if it's not the victimized player's first choice of things to happen. Who's to say a demon didn't possess them at the time?
* Permanent level drain is included in this rule set. Be prepared.
* If you want to play a class from a different OSR ruleset or different iteration of D&D up through AD&D 2nd Edition, it's not hard to convert at all. Let's do it. I don't care if it seems "overpowered". I'll adjust things accordingly.
* No cursing at the table, please. We want kids to be able to play with us if they show interest. Likewise, treat everyone with respect. I am a father of two girls and a stepfather of two boys with autism. Play like they're at the table and set a good example. Also, help new people to learn. Do not deride them as "newbies." That is despicable.
* There will be no rules-lawyering. If you have an issue with a ruling, we can discuss it after the game. If for some reason, I make an error in judging a situation, we can rectify it afterward, up to and including resurrection of a character if necessary. Anger over a game won't be tolerated. It isn't that important. It's a game.
* I am not "out to get" your characters. At the same time, I shall not fudge a die roll. Approach encounters with caution, as well as dungeons and wilderness in general. I promise to be fair. If it kills you, it kills you, and if you kill the bad guy, it dies, too, no matter how much planning went into it. If you hammer the final enemy with one nasty spell and he's down, then I didn't plan properly for what your characters can do.
* Dungeon environments are apt to be an "underworld" environment where the world works a little differently and perhaps unexpectedly. Doors lock and open on their own sometimes and are constantly sticking, closing behind you constantly after a few turns, making the characters' lives difficult, and all or most of the monsters can see in the dark down there. It's the nature of the place. It hates you and doesn't want you there. It has a mind of its own, or at least it seems that way.
* I expect the players to use everything at their disposal, including the depths of their imaginations, and bring it to bear against their antagonists. Likewise, the bad guys will have plans in place to deal with adventurers and thieves. If you outwit me at the table, I will enjoy it and reward good, skilled play accordingly.
* I tend to dislike the whole "speaking with funny accents" aspect of roleplaying and other cliches of this type of game, since I find it grating. I tend to narrate like a story teller rather than act. It's up to you what you choose to do.
* This is a game of exploration. Be prepared to map as you go. It doesn't have to be 100% accurate, nor is it ever expected to be unless you make it clear your characters are going to take the ridiculous amount of time it would take to measure out each chamber, alcove, and tunnel. All you need to know is how to get out of the place you're exploring and where unexplored areas are located.
* I'll let you know of any house rules that we'll be using, like the famous Shields Shall Be Splintered rule, or if we will deviate from the current ruleset in any way. For instance, in BECMI D&D I tend to drop the skill system completely, or award them as special abilities.
* This is a table of respect and fun. Be respectful of the life experiences of others. If the adventure is making you uncomfortable, let me know somehow. We want to have fun, not remind you of something that happened to you that was no fun at all. At the same time, let's all have a sense of humor and not be overly sensitive. Again, we want to have fun. For my part, as far as the themes encountered, the game will be the equivalent of a PG-13 movie rating or less. I won't touch upon certain taboo subject matter for the sake of shock or horror.