Some things to cover before the discussion even starts.
Remember that the game system is not an attempt to 'exactly simulate' the game world. That is, what the game system says happens, happens, but has very little to say about
how it happened.
So, for example, the system can say something like:
Wizard Alice throws a fireball at Goon Bob, He takes 6 stress, since he only has 4 stress boxes, he has to take consequences if he doesn't want to get taken out. So he takes the mild consequence of 'sprained ankle' and fills in his fourth stress box.
Huh? how does a fireball cause a sprained ankle?! Because the GM says, "Bob barely manages to dive out of the fireball in time, but he lands badly getting out of the way and sprains his ankle."
So, what determines whether the 'text' of the consequence is an acceptable result of what the dice say? Whatever the people at the table will put up with.
Next, being 'taken out' does have to mean 'death'. Expanding on the previous example, Goon Bob is just a goon, and will only mechanically take a mild consequence, and will get taken out after that. Does mean that Bob is really a bag of blood and gore under high internal pressure, just waiting for a good, sharp poke to explode and spray gibs everywhere? Not really. Bobs take out could just as easily involve him running away, or giving up, or something else like that.
Here's another system disconnect that you need to pay attention to: 'weapons' designed to take out narratively important people look a whole lot like tank canons when you turn them on people who don't have consequences.
For example, when I first wrote up
Jing Wei I gave her a 'tazer ring' enchanted item that was power 7. Why? because I wanted to be able to bring down someone 'important' like Victor Sells with only a few shots. And the fact that sells has plenty of consequences to soak up attack from the tazer ring means that I'd maybe also have to spend some time hitting him over the head or something too to actually take him out. But when the same ring gets used on a goon with 3 stress, some people might complain about me using a power 7 weapon (even bigger than a hand grenade, fer cryin out loud!) and expect it to not instagib normal people.
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So, finally, on to the actual discussion. The first line of defense is to have a discussion around the table to exactly what kinds of consequences and take out results are acceptable what sorts of attacks. Another thing you can do if you want to constrain the power someone can throw out in a scene is to compel them.
For example, Wizard Alice normally calls down massive gouts of fire from heaven upon her enemies, but somehow, people are only ever knocked out by them. If you wanted to contrain here power for a fight, you could try offering a compel of her high concept, something like "The gods of death are watching this fight closely, any massive and/or lethal looking attacks are probably going to kill someone."
Then, the players can accept the compel and have to come up with something creative, or they can just buy out of it and throw fire from heaven around as usual.
