Some questions to answer in your social contract:
Can my character die without my consent? In D&D (and most action-based games) the default answer is 'yes'. Subquestions to ask: Will I get a hint that I'm in serious danger? (In 4e you usually won't need one... it'll be obvious that you're low on surges and survivability.) How likely is this? (Players in my games know that I will kill characters, but it hasn't actually happened more than a couple of times in the last 10 years. The possibility is enough.)
Can my character die as a result of a single failure? Are there effects which kill or disable you at once if you fail a save, regardless of your state of health otherwise? In D&D 1 the answer was 'very much yes' and each edition since has been less so - a very good thing IMO. 3e in particular made a deliberate effort to reduce this, 4e more so - so in 4e you can say 'no' with no real change.
Can my character die as a result of another PCs actions? D&D mechanics assume a social contract in which the group are all on the same side, and in 4e this assumption is very strong. But good stuff can be done with characters that are mostly on the same side; conflicting goals -> character development -> entertainment. So: Might another PC abandon the group during a fight?
In particular, especially in D&D, cover can other players kill my character? as an entirely different question from whether the game world might. D&D 4e assumes 'No' as a default here, and 3e assumes 'very unlikely'.
[Edit/added, thanks to Valadil:]
Can my character die when I'm not present? If a player can't make it to the game, does their character vanish, or get played by other players, or by the GM? It's demoralizing to have a character killed or permanently damaged when you're not there. (Our group uses: GM tries not to kill the character when the player isn't there, but only if players refrain from using the PC as invulnerable point man... PC actions are by group consensus, but the PC does nothing especially heroic or dangerous. Basically, a no-score draw.)
Can my character be resurrected once dead? In D&D the default answer is 'yes, at a cost', if you're high enough level. In 4e the default answer is 'yes, at a not-high cost', especially in LFR. (Look at these rules; in a home campaign they have implications for the game world. We play LFR adventures... but the LFR resurrection rules seem too cheap to us; we multiply all costs by 10.)
Do the rest of the group have to try and resurrect me? In D&D, a regular party might even have in in-game contract for this one. Be sure it specifies who pays!
At what level does my replacement character join the group? (Our long-standing D&D and Star Wars answer has been 'at the bottom of the level below your previous character's level, plus some bonus xp based on how you roleplayed the death scene'. Another common answer is 'Same level as the lowest-level character in the group'.)
What non-base abilities/equipment can my new character have? In D&D in particular, magical items are the issue. 3e and 4e assume a certain level of magical item power for any given level of character; al cash value by level can be an acceptable guideline but not a dramatically brilliant one. In D&D for mage characters, you also need to consider 'How many spells does a new wizard know?' (in 3e) or 'How many rituals do I have?' (in 4e).
Adventurers being adventurers, and depending on your group's play style, you may also need to ask if my character dies, do the party loot the body for useful magic? (GMing note: Having an unknown relative show up and lay claim to the deceased PCs possessions is a fun way to cause trouble for item-oriented groups... especially if they had a mission-critical item at the time.)
The Living Forgotten Realms rules cover all of this well and are worth a look, but don't use them as a model for a regular group. They're designed to handle a situation in which you're playing drop-in convention games with random strangers and want the GM to recognise existing characters. They do a decent job of examining the kind of question that can come up.
I can't give you a full list of the occurrences, because they are sprinkled throughout, so I'll focus a bit on why those two examples are expressly called out as deciding them before play.
The reason these are called out is because they have the potential to hurt feelings and cause heated discussions at the table.
Lawbreaker example
First example being the lawbreaker rules. If you break a law of magic, you immediately gain a stunt that costs 1 refresh: Lawbreaker [law number]. Many wizards are probably skirting the line with only 1 refresh remaining, so if they break a law, they are immediately in NPC territory. Thus it's a really really good idea to hash out in your game what precisely constitutes a law violation.
For example, if you burn a building to the ground that might have humans in it, and some of them perhaps died in the crossfire, does that constitute a violation of the 1st law of magic?
Now, you can probably get away doing this during play, so long as you warn the player that he/she is about to become a lawbreaker, and that no further warnings will be issued, yadda yadda. However, I feel that everything goes more smoothly if you hammer this one out before play. Even if you do this, though, you will need to make sure the group is okay with a single player being the final arbiter as well.
Death as a result of being taken out
This is less important, I think, as you generally as a courtesy should inform the players that a given conflict could wind up being lethal before the dice are rolled. That gives them the chance to offer a concession before they, you know, die. You might want to lay out some ground rules with your players first like: "I'll let you know if you think you're likely to die in a combat." Or "I will not inform you if you are going to die as a result of combat, so please keep your wits about you." That way you don't run into any issues when some of Johnny Marcone's thugs come to beat their heads in with baseball bats.
Some other examples
- City and Story themes: These are explicitly defined by the players in the first session, and thus have to be done before play begins.
- How nasty a sponsor for sponsored magic is likely to be: That way the player knows what they're getting into...or that they really have no idea what they're getting into.
Best Answer
While Lines are about themes that are completely off limits and Veils about "censored" content, when you are putting something in normally you are dealing with a binary situation: that content wasn't limited by veils and lines, so anything about it goes.
I, however, use a more "poetic" way to deal with that:
Vices and Sins
Examples:
Wait, wait. Hold on. How are Veils and Sins different? How can I use Veils, Lines, Sins and Vices together?
Well. Both Veils and Sins present content in a "Safe Mode" way. However, while Veils are carefully drawn across content to not shock sensitive people, Sins work in a way that permit you to expand your content on demand. If something is a Veil, it establishes a maximum level of detail. If something is a Sin, it establishes a minimum level.
Compare these examples:
So, you can use all four together: