Everybody has to agree to the rule for it to be used - the GM, as the organizer of the game, typically brings them to the table, rules are about universal agreement.
This is sort of what I said, but coming from the opposite end. I do feel that if the players don't agree, then some sort of compromise has to be reached. This is the "universal consensus" I spoke of earlier, but on a smaller scale. I still don't think that all gamers can reach an agreement without making some holy hamburgers, though. I once wanted a wizard that could wield a greatsword. My DM allowed it, but I had to put NWPs into it, as well as all of my weapon proficiencies while I "learned" it. At around 6th level, I think, I finally bought off all of the THAC0 penalties.
Otherwise you get deadlock ("Roll Reflex Save." "No.")

That's just funny. One of my friends is just THAT contrary.
So yes, all people involved have to be "OK" with a ruleset to have a good time. But people can say they're "OK" with a ruleset, and abide by the rules, and still have a boring, uncomfortable time.
This refers to what you say later, about friendship, and I've been there, too. I think, though, that most people are going to tell you what makes them happy about a system that's being run. And, I can usually tell when people are liking my games. I try to shake things up a bit, when it seems like they're not.
I've had this experience when(nominally) playing D&D - the DM looks at d20 rolls, thinks about the PCs and monsters as they've been presented so far, and declares results based on that instead of doing the math.
I have actually attempted this on two seperate occasions. Neither worked very well, but the situation was screwy enough in game that nobody seemed to mind. We didn't get much roleplaying done, but we laughed alot. It was kind of obvious that I was doing things on the fly, though.
If that experience was generally true of a system (i.e. people only had fun when a major rule was "we're mostly ignoring the rules") I'd say that system was badly designed. I'd even say that in the face of the gamers that say "We play that, and we have a blast!" and I'd certainly say it to the GM that claimed the system was great while his players stuck to the game out of friendship or loyalty rather than fun.
On both occasions, it was completely my fault. The first time, I was unprepared, and it started showing early-on. The second was something of an experiment, and we ended up with alot of meta-game playing, like the players starting to do like I was, and using player knowledge to make character decisions, etc. It this circumstance, they went along with me and it actually helped with the hole I'd dug for myself. It was kind of an unspoken agreement to play that way, right then.
Best practices is about getting effective rules that deliver what a given group of gamers wants out of a game. There's lots of room for variety in there - but that's not the same as saying that every group is getting it right, or that every system gets it right for at least some group. The "I'm OK, you're OK" attitude that I take issue with is about this kind of scenario:
System X is billed as dark horror kind of fantasy game. The setting has a lot of wizards that tend to cast magic every round of combat. Each time a caster casts a spell, there is a 10% chance that all PCs in the party turn into a bowl of green onions. This is just one example; the system has a pandemic of green-onion related lethal random events.
Gamer A: System X is crappy; I tried to run a dark horror fantasy game and it just ended up silly.
Gamer B: I like system X, because it's the magic system is really cinematic! I ran a dark fantasy / horror game and it was the bomb!
Gamer A: Oh, come on. The author clearly had a green onion fetish, and it's ruined the game. Not to mention the lethality.
Gamer B: Hey, if my group liked it, who are you to judge the game?
Gamer C: I guess it all comes down to preferences.
I have to ask: why green onions?
Aside from that, what you seem to be saying is that preference has to be subjective, which is part of its definition. Gamer C just seems to be placating the other two, by making them each think they are right, perhaps to avoid a conflict. The problem in the situation is that the conflict needs to happen, so the other two can get a better handle on where their lines are drawn, and Gamer C is preventing that with his/her fence-straddling. If they don't work out what system they want, the three of them will just sit there, doing nothing.
I still think that preference has to play a major role in most choices, especially in a hobby like ours. It determines the kinds of characters we play, the rules we use or change to suit us, who we associate with---my last offer of a gaming group seemed like a bunch of bad people, so I bowed out---at the table, and a bunch of other stuff.
You seem pretty strong in this opinion, so I probably can't convince you. I do agree, though, that the "Gamer C Mentality", while alot nicer than many people are about their preferences

, is not constructive or conducive to progress in finding out what people want.