So I've described the "job to be done" as "counting individual acts toward resolution of an arc" and "ways to say when enough has been done toward a goal." I might be misreading your post to have a tone of disagreement: if that is on the matter of scale, I would argue that the fundamental job done, and consequential distinctions between members of this family of mechanics, don't turn on that.
Well, I interpret SCs and Clocks as being more BINDERS ON THE GM than anything else. The problem I see with 5e (as an example) is that there's really nothing like that in place. Not only does the GM get to present any sort of situation she desires to the players, but she also gets complete arbitrary say over what the win cons are. Its not even a 'game', it is simply "when I feel like I've made you roll however many dice I feel like, then I'll tell you if you won or lost and what the consequences are."
So, 4e SCs in particular, exist to defeat the above. The GM must declare "this is a level 5 complexity 4 challenge" and from that moment onward the win cons and stakes are set (I'd say the players may, informally in 4e, have ways to up the stakes, but the GM is bound). I don't think 'momentum' as such is central to this, though the way you have just described it above may be consonant with what I'm talking about. Still, our evaluation criteria are probably rather different, I'm not sure.
BitD clocks are pretty similar to SCs IMHO. In fact I suggested that they were filling the same role way back when BitD first appeared, but I got told I was out of my mind. So, I dunno, maybe we agree in some fashion.
The 4e DMG2 has the most to say on the scale and breadth of skill challenges. "You can even use a structure challenge as the framework for a whole section of an adventure, or even the entire adventure." The "War By Other Means" case study "represents the player characters’ efforts to oversee and perhaps influence the negotiations as they try to keep matters from devolving into violence."
But compare with the looser structure of momentum points in L5R, which counts only successes. Also counting failures as 4e does has consequences for play.
Sure, as does the push/pull architecture of BitD clocks, which can count in either direction. I think there is considerable room to consider the merits of each design and which specific sorts of game/play it serves best. I felt like the SC is a bit more 'solid' structure than the clock, which is maybe best for highly fluid situations where you have direct opposition.
You mention tactical and operational concerns, and strategic... a wide range. As examples, Long-Term Project and Faction clocks can span sessions.
I would differentiate BitD clocks from momentum points and SCs on the basis that normally only player characters can earn momentum points or SC successes/fails, but BitD clocks are generally driven by other things. In terms of structure and potential, though, they're part of the same family of mechanics.
Yes, BitD allows for clocks that are imposed on the PCs and largely track things that are somewhat out of the control of the PCs. BitD is a game where the character's are weak and oppressed. Its about 'fighting the system', whereas 4e is about big heroes doing epic heroic deeds. So they have different needs in that sense.
Good point. The way dangers trigger grim portents that herald the impending doom is a kind of clock. They're looser than the other examples I chose, and while player characters will no doubt form goals relating to them and ultimately drive them, to my mind they stand as an outlier. Still, they do exemplify the general job to do - when has the badness gone far enough to trigger the end?
This could be looked at in terms of a history of Narrativist game design. Fronts and dangers were devised by VB for Apocalypse World. Some other PbtA games adopted this design, and then 4e introduced SCs as an encounter mechanism which was extrapolated a bit to cover higher level concerns (though I would say not much used in that capacity). BitD then introduced clocks as a bit looser but analogous construct. AW 2e has applied that design to threats, though AW didn't really need a 'clock' to drive its more tactical level of play (I guess you could map threats at that level though if you wished).
Momentum mechanics speak to a contract between participants as to what equals enough. When have we done enough to navigate through the Forest of Neverlight? When have we done enough to find the Mastermind, persuade the Duke, and so on. They constrain and compel... in partricular constraining and compelling GM. Desiring to do that is one sign of a neo-trad design: it's not just - no rule zero - it's here's some boilerplate for your negotiations.
Sure, but I'm not sure I'd attach all that to a 'culture of play'. "Neo-Trad" to me signifies a high degree of focus on player-curated character arcs and a focus on enacting player-envisaged plots and action based on that. Certainly a clock/SC could be useful there in terms of structuring how the GM approaches enacting her side of that. However, I think it would potentially be equally useful in trad play as a way of constructing a gamist structure for challenge development. I'm not sure how useful it is in a more Sim sense, but undoubtedly there are situations where it is a useful tool in the box.
So, I see these mechanics as being broadly applicable to a wide variety of types of game, though not every technique will work well in every case (clocks might, for example be pretty awesome in classic Gygaxian gamist play as an alternative to things like wandering monsters, but I think SCs would generally be a bit rigid there).
