In the second case, the mechanics constrain the imagined game space, effectively becoming the physics of that fictional reality.
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2nd case: Come and Get It affects all creatures, 'cause that's what the rules say.
I wanted to follow up a bit on Hussar's post #427.
I think that to say, in a game like 4e, that the mechanics become the physics of the fictional reality is a little unfair - it's hard to find a non-tendentious analogy, but I'll try - at least some practitioners of formalist or non-representationalist styles of art might reject an attempt to characterise what they're doing as just some special variant on representationalism - perhaps the representation of certain abstract concepts. The artists have set out to repudiate representation, not to represent strange things in a strange way, and a description of what they're doing that already seems to introduce the judgment that they have failed might be one they reject.
Likewise, for those who see the mechanics as primarily operating at a metagame level, and setting constraints on permissible narration of what is happening in the fictional reality, it is a bit tendentious to say (in simulationist fashion) that their mechanics are the physics of that fictional reality. Because what those players tried to do is have a game where the physics of the ingame world are whatever they are, and the players have a duty to fit their narrations to those physics, but the players
also have a duty to fit their narration within the parameters determined by the mechanics.
(Is there a mutually acceptable, non-tendentious way of describing differing play styles? I would like to think so, but it's certainly not a given. Anymore than we can take it for granted that there is some non-tendentious way of describing what it is that a modernist likes about modernism and what it is that a romantic likes about romanticism.)
Can the sort of RPG design I have tried to describe above cause problems if the two sets of constraints come into conflict? Of course. But two factors mitigate the practical consequences of this: (i) the causal constraints on any imagined fictional situation are normally pretty loose, allowing a lot of free narration to plug any gaps - for example, it is almost always feasible to narrate an unexpected gust of wind to explain a surprsing outcome of an attempt to jump, because in most cases the imgained fictional situation doesn't have its details specified to a degree of precision that would exclude wind gusts; (ii) if the action resolution and encounter building guidelines of the ruleset are well-integrated, then comparatively few situations will be ones where corner cases will arise (in 4e, for example, the game
simply takes for granted that epic tier demigods will not be enaged in life or death situations involving ordinary orcs, mundane locks or 10' wide chasms).
As for Come and Get It, that power tells us nothing about the physics of the gameworld. What the power does is give the fighter player a 1x/encounter token that says "When you play this token, all the foes within 3 squares move adjacent to your PC. You and/or your GM are free to work out whatever story explains this." It's like a Fate Point or Hero Point or Luck Point that exists in many games.
Typically, when the polearm fighter in my game uses this power he chooses to narrate some story about his deft use of his polearm and/or the biting character of his insult of the gnoll warrior ancestors. When something tricker is required I'm happy to help him out with his story. But his repeated use of Come and Get It doesn't reflect on the physics of the gameworld. It's a metagame technique.
(EDIT: I don't know if this is quite what Hussar means by a metagame construct. I think it is, though. Where I think I differ from Hussar is this: while I agree with Hussar that spotting the difference, at the table, between a metagame heavy game and a simulationist game might be hard - the two games might look very similar - I nevertheless think that the difference in the purposes and self-conception of the players is a real one. And conversations like this - where, rather than playing, we try to bring to mind our purposes and our self-conceptions as RPGers and explain them to our fellows - make those differences in experience become all the more salient. This is what I was getting at upthread when I said that the move from metagame heavy encounter desing to metagame heavy action resolution might be one step too far for many ENworlders.)
(BONUS EDIT: If you stumble into a recitation of the "what a piece of work is man" soliloquy from Hamlet, it might sound the same whether it is part of a very sincere and purposeful performance of the play, or is instead the ironic culmination to a story (at least arguably) about an aspiring actor wasting his life, as in the movie Withnail and I. The fact that you can't tell, just from experiencing the recitation, which purpose it was serving, doesn't make it unimportant either to the performer, or to the other members of the audience, that it was one thing and not the other. In my view the same point, mutatis mutandis, applies to episodes of play in an RPG.)