I answered "No." I recognize the option of choosing "yes" and fully understand why people would prefer that, for example. It's perfectly viable and fun, particularly if everyone's on board.
However, I do not well understand how people cannot comprehend the "No" option at all. There are means to play by which GM fiat can be avoided. Some of these means are unconventional and quite different from standard play. It's not everyone's cup of tea; again, it's a matter of preference.
I will attempt a quick stab at what I mean: I understand GM fiat to describe the means by which uncodified decisions are made. By default, most game groups defer to the judgment of the GM. This is fine, but not required nor "definitional" to role-playing games.
Here is an example:
Suppose your D&D characters are after some kind of bauble that's important for the adventure. Let's say it's "supposed" to be inside a castle or something. You're not sure where it is, so you enter the castle and start looking. You enter what appears to be an "important" room -- the castle lord's vault, let's say.
You find a chest. You tell the GM the following: "Ok, I think the bauble's in the chest. I'm going to use my Rogue's skills to pick the lock -- I check for traps first, of course (roll). Ok, now I'd like to open the lock (roll)."
At this point in time, ther are a number of ways this can happen:
First, the bauble COULD be in the chest. The GM knows it, you don't. If you open the lock, voila! The bauble is yours. Neat. No GM fiat. The GM didn't decide anything "arbitrarily." You had a challenge. You overcame it with a skill roll. Cool beans. This is pretty normal play.
Second, the bauble could NOT be in the chest. The GM knows it, you don't. Even if you open the lock, no bauble. Boo hoo. But, it isn't aribitrary. The GM just had a scenario created, and this chest didn't contain the bauble in his notes. No big deal, and again pretty normal.
Third, the bauble "WAS" in the chest, but the GM decides right then and there it's too easy for your group to get it at this time, so he "moves" it. Boo hoo, no bauble. THIS IS GM FIAT. (MY preference: I think this sucks!)
Fourth, the bauble "WASN'T" in the chest. But, given the circumstances (maybe, for example, it's getting late in "real world" time and the adventure needs to be over soon) the GM decides right then and there to "move" it into the chest. Yay. You get the bauble. THIS IS GM FIAT. (Again, MY preference: I think this sucks!).
Fifth, the bauable MAY or MAY NOT be in the chest. The GM doesn't know and neither do you. But, you both decide that it'd be a pretty cool place for the bauble to be, and entirely plausible given the environment and adventure. SO, the in reaction to your annoucement, the GM says, "Ok, cool. If you make this skill roll, plus another search roll, you'll find the bauble." Sweet! You just convinced the GM to agree to your terms. THIS IS NOT GM FIAT. (MY prefernce: I think this rocks!)
So, you can see that GM fiat really boils down to who makes decisions, particuarly "on the fly." And, you can further see that there are means -- whether you prefer them or not -- to remove GM fiat from the game entirely. YOu and your group can choose OTHER means than GM fiat to resolve decisions. One way is to set it up so that you make a request, the GM approves the possible outcomes of that request, and the dice decide what happens. That;'s is pretty cool stuff!