I'm not sure I've completely followed [MENTION=6696971]Manbearcat[/MENTION]'s analysis of the instance of play that he provided as an example, but I thought the decision was oriented towards the fiction: the player, confronted (via the GM's narration) with a threat to his/her PC's interests/desires, has to choose how his/her PC responds to the threat.I'd like to focus on point c. In what way does the player face a decision point?
That choice will also have mechanical ramifications (eg it will determine which skill is used to determine whether the choice has good or bad results), and the player is expected to be keeping those ramifications in mind when choosing.
I think why it is being described as "a legitimate decision point" is because the choice the player makes at this moment, in response to this particular prompting from the GM (by the narration of a threat/obstacle) will determine a skill check. And the outcome of that skill check will have (i) a definite mechanical significance for determining whether or not the player (and thereby the PC) gets what s/he wants, and (ii) a definite fictional consequence which will shape the parameters for the GM's next episode of narration. (Up until the encounter is resolved one way or another.)
Based on [MENTION=6696971]Manbearcat[/MENTION]'s posts upthread, here are two examples of what I think he would count as "non-legitimate decision points" (to coin a phrase).
The first would be a player in AD&D 2nd ed saying "I want to use my Survival NWP to find a shelter", but there are no mechanics that connect that check to the Natural Shelter table. In this case, the only conduit between the player's action declaration (which is what it looks like, at least on the face of things) and the resolution of that declaration is GM fiat of some sort to bridge the two non-integrated mechanical subsystems. So the player's decision becomes subordinated to the GM's fiat.
The second would be a player, in a free-roleplaying style, describing his/her PC describing cutting brush and gathering leaves for beds, checking the cave for hibernating bears, etc, and there is no determination of whether the description of what is done is adequate or not to provide the children with proper shelter until the GM is satisfied that enough has been done. Again, I think this would be seen as the player's decisions being subordinated to GM fiat.
Also, I'll connect this discussion to something [MENTION=6668292]JamesonCourage[/MENTION] has queried upthread, namely, the GM's authority to narrate complications. In Burning Wheel, the rulebook indicates quite clearly that, before the dice are rolled to resolve a check, the GM must state the consequences of failure. In the Adventure Burner (which is BW's analogue of a GM's guide), Luke Crane explains that he typically doesn't follow that rule because (i) the consequences of failure are often implicit in the scene as framed, particularly once the details of what the player is having his/her PC attempt have also been worked out, and (ii) his players trust him to be a fair GM.
Both in 4e GMing and in BW I alternate between stating clear consequences, and leaving it implicit, based roughly on how strong I think Luke Crane's factor (i) is in play. (I think factor (ii) is always in play - my players trust me - but sometimes it is still important for everyone to be crystal clear on what exactly is at stake.) I strongly suspect that [MENTION=6696971]Manbearcat[/MENTION] takes a similar approach.
Even when consequences of failure are left implicit, I don't think illusionism is very likely or feasible. For instance, imagine if, in response to a failed check in the shelter skill challenge [MENTION=6696971]Manbearcat[/MENTION] were to frame the player into a social confrontation that (given the PC's stats) s/he probably can't win (eg "As you check the cave for its safety - Oops, failed Dungeoneering! - you fall through a hole in the floor and are surrounded by half-a-dozen angry goblins, crossbows pointed at you). If it had already been established that there were goblins in the caves, and that some of the caves had concealed holes in their floors, then this might be reasonable, and not particularly illusionist: the player, having a general sense of what's at stake in the caves, has decided to try her hand at Dungeoneering, and now it's failed and she reaps the consequences.
But if it hadn't been established that their are goblins in the caves, that the caves are treacherous, etc, then this is hardly illusionistic either: it's quite blatant! Depending on the table conventions for ultra-hard scene-framing it might be acceptable or not, but I don't see any illusionism taking place.
This is all a consequence of their being a "legitimate decision point" (as I understand that phrase). The occurrence of that point, and the mechanical procedures around it (check declared, made, and passed or failed), means that the GM's exercise of narrative authority - whether done well or poorly - is quite overt.