I think the FUNDAMENTAL area where we all are not on the same page is in terms of the purpose of the rules. The rules of D&D, mostly, seem to be intended to act as aids to a GM in determining what happens in the fictional game universe. Some of the rules (IE levels, hit dice, classes) may be 'game constructs' which don't directly correspond with things in the game world, but they are still aimed at telling the GM how that world works. That is the purpose of rules in a 'Gygaxian' system.
In Dungeon World, for example, the rules are ENTIRELY about how to manage the STORY. None of the rules of Dungeon World relate concretely to things in the game world, although some of them are pretty close (PCs have an inventory for instance, though at times a GM might 'hard move' part of it out of existence, and this would not be a 'dick move' as it would in say AD&D. This is why it is impossible to 'break' Dungeon World, at least in the way
@Lanefan seemed to be proposing. To 'break' DW would require something more like spitting in the GM's face... Since ANY action a character is going to take will be related to the fiction in some way, there's no action that the rules don't cover, because they cover "adding to the fiction", not "actions PCs take."
Well.... I don't know that the 4e SC system is the epitome of resolution systems for fiction. I have called it out and mentioned its function several times because we all have some understanding of what it is, and yet at the same time I suspect many people never really understood what its role in (at least story game people's) 4e play was. There are MANY other mechanisms which accomplish similar things. DW moves, BitD clocks, FATE's leveraging of traits (also shared with several other similar systems), etc. I have a super lite system that I have used, PACE, which uses the spending of tokens as a sort of 'cash' to alter the fiction. There are lots of ways to structure rules which work on the level of narrative structure vs in-game process. Most games mix the two to at least SOME extent, though PACE is pretty much entirely on the "rules are about how to construct the narrative" side...
But to answer your questions: Yeah, I can create SCs on the fly. Admittedly there is some element in which the GM is going to exercise judgment. However I will note that I've run my SCs by, essentially, the 4e Essentials RC version for a long time now. While that means that the GM gets to decide "primary and secondary skills" the PCs also get 'advantages', which they can use to either erase failures, or bend the conditions. I interpret these as chits to be used to inject their own narrative elements. So a player can invoke an advantage, and invent some situation in which they suddenly get to use their best skill, even though it isn't a part of the SC (IE, we're negotiating, but all of a sudden I use my DEX to take a check to skewer a deadly scorpion which was about to sting an important NPC, an assassination has been thwarted, SUCCESS!).
I go much further in my own system. Players can use my version of "inspiration" to leverage a character trait in order to alter the fiction. Later they can alter the fiction in a way which adds an impediment to their goals to get that inspiration back. Also the purpose of 'Rituals' in my system is to simply change which ability you use for a check. A 'true seeing' ritual would substitute Arcana in place of Perception, for example. When casting these rituals (not all are magical either,
@Garthanos!) you can also pay a supplementary cost to pass the check automatically. Of course, the resource costs for this are usually pretty significant, so its actually a way of indicating what is most important to your character, with the intent being you're going to lose out somewhere else (IE because you are now broke or very tired, etc.). By the time you layer these sorts of devices onto the system it becomes pretty fluid in practice. Its main purpose, as I have stated, being to nail down what the criteria for closing out the challenge are. Otherwise players are totally at the mercy of the GM to say if and when they have won or lost.