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.
I don't have my books handy, so I can't quote rules for you, but here's my recollection based on several years of playing Fate:
Players can narrate their own actions. And they can answer questions that you ask them - filling in blanks and taking a portion of the burden of narration from you.
But when they want to create facts to their own advantage, they usually have to earn it. Earning it means succeeding in rolls or spending Fate points or sometimes both.
For example, if a player wants to to put the aspect On fire on something, she can't just make it so by saying so. She has to Maneuver Create an Advantage.
If a player wants to change an existing fact, that has to be earned, too, if it's important. The simplest example is this: A door is locked shut. The player wants it open. To make the open door a fact, she'll have to pick the lock with a roll, get the key with a roll, or bash it down with a roll. You and your players have an intuitive understanding that this is so.
Aspects can create facts for players, but they have to be powered by a Fate point. For example, I can say, "I'm invoking Better late than never to put myself in the scene now!" presuming I have an aspect "Better late than never" and the action has already started (so that I'm late) and also assuming I pay a Fate point for it. But the GM can give me a Fate point to cancel that invocation.
Your player created a fact out of nothing - he got to narrate how he succeeded at his roll, but since everyone's job is to make everyone else look awesome and his narration is squashing your awesome with "meh", you are fully within your rights to veto it.
Here's the only thing I would have done differently and would suggest that you do differently too:
As you say, "nah, sorry, but that didn't happen", hand over a Fate point. You are saying, silently, but concretely, "I am taking over narration here. Sorry, but in exchange, I am literally handing over narrative power that you can use later."
Best Answer
Now there are two sides to that coin.
The good part is that Fate is an awesome system when it comes to player-vs-player games. Sometimes the narrative leads to a situation when two player characters find themselves in the opposite sides of the fence. The typical fate mechanisms like invoking and compelling, coupled with the fate point economy, lends itself to a self balancing game where the players take the lead in creating a story and doing the usual GM job of creating the conflict all by themselves. In a game where the players have internalized how Fate works, all you have to do as a GM is sit back and enjoy the show, and occasionally throw in a wrench by playing the rest of the world just as a player plays his character.
The bad part is that because the Fate system is a narration system as opposed to a simulation system, in which some points are deliberately left open to discussion and consensus on purpose, it is easily exploitable by powergamers who see freely bendable opportunities everywhere.
I have played a lot of games that turned PvP using Fate, and whenever there was a powergamer in the group that attempted to dominate the game, I used this three level approach to get things going smoothly.
And of course, if he insists on ruining the fun by other means, kick him out as usual. Fortunately, I never had to come to that with Fate. My regular powergamer had a taste of the third level for a couple of times, getting brutally killed by another player's character in one of them. He has now learned to enjoy the game at the first level and provides formidable opposition for other players.