Thasmodious
First Post
But if the rule in question works in a different way - for example, if its function is to confer a certain sort of narrative authority on the player who has used it - then it seems to me that the GM doesn't have any special privilege of suspending or altering the rule. The whole point of this rule, after all, is to confer on the player a privilege that, in a mainstream game, tends to default to the GM. For the GM to purport to take that privilege back at the precise moment the player in question uses the rule looks to me at least prima facie like an unjustified breaking of the rules.
This is at the core of what I've been arguing. You just stated it much better than I have managed to so far.

I would add that the narrative control ceded to the player here isn't really any different than the narrative control the player has always had. The powers of 4e are, typically, contingent on a single roll, rather than a series of rolls or the use of charts, or a back and forth dance of rolls to produce a single outcome (like 3e grapple). Knocking something prone because your chosen power does that is not really grabbing some special control that PCs didn't have before. Spell effects, damage, feats, whatever, these are all examples of the player telling the DM what is happening to his monster. It's packaged a bit differently in 4e, expanded a bit, perhaps, through sheer volume (since most powers do damage +). But at the end of the day it isn't functionally much different to say to the DM:
"Thorax has hit your beast for 7 damage."
or
"Thorax has hit your beast for 7 damage and knocked it prone."
The arguments against knocking a snake prone have been pretty soundly defeated. At the end of the day, snakes do not have an immunity to special conditions in the system, so to deny the player the ability to apply one requires a pretty tight case. In this situation, the case has been solved, repeatedly, by numerous examples.
IS there a situation in 4e where a power applying a condition to a creature not already immune to such things simply can't be rationalized? I don't know, I haven't found one. Prone oozes, petrified stone golems, blinded bats, yeah, I can rationalize those, as player or DM.
To flesh this out with a concrete example: some event, narrated by the GM and resolved and adjudicated in the usual fashion results in the death of a PC. The player of that PC then declares "I'm spending my last Fate Point to save my PC's life". The GM says "There's no way anyone could have survived that!" The player then gives some contrived and or improbable, but by no means impossible within the fictional context, explanation of the PC's survival. Should the GM nevertheless be able to veto because s/he doesn't like the story? Doesn't look like it to me.
It seems to me that several of the people arguing the other side here would say that absolutely the DM could veto the Fate Point. I don't agree and you very well sum up the reason why:
If you GM a game with Fate Points, you've agreed to cede narrative authority at certain key points to your players. Part of playing the game, then, is putting up with their Fate Point usages and incorporating them into the overall shared fiction.
Exactly.