We all like the different styles to varying degrees, but at least a little, and have played games that were extreme in each vein, and enjoyed them. This is where "creative agenda" makes some sense, in that it focuses on what you want right now, as opposed to "what kind of player you are."
So I suppose in a strict creative agenda sense, we are people who are changing our creative agenda during a session, sometimes rapidly, and picking up cues from the other players in order to shift seemlessly.
I know it's official doctrine in the Forge/"big model" approach that multiple creative agendas can't be pursued simultaneously, but my experience feels, in practice, a bit closer to yours. To put it in big model terms, the distinction between creative agenda (= the end of play) and techniques (= the means to that end) can in my view sometimes get a bit blurred, especially because the notion of "period of play over which payoff is received" is itself so open-ended.
Talking about 4e in particular: at least some of the engagement with thematic material/expression of thematic concerns is going to happen during combat. But this is also where the techniques used in the game make system mastery and understanding important. And whenever mastery of the system is important, some sort of "step on up" can come into play, even on a modest level, bringing with it the possibility of competitoin for peer esteem. Is this always subordinate at all times in all players to the overall narrativist goal? I'm too involved in the situation to judge this.
I'm more comfortable in saying that prepared modules/adventure paths in what seems to be the received style is something I'm not interested in GMing - so whether or not my players would enjoy it, they're not getting it! I have enjoyed running more sandboxy games in the past, but I think most of my players are not that interested in the "exploration only" approach, and I tend not to be able to help myself from introducing the thematic prodding and poking, so my sandboxes tend to transform into more situation-focused games over time. Recognising that, these days I tend to prefer just to start at that point.
OK, lets just blow the doors off this joint!
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Lets take this building example. Now, suppose you'd defined when you added this element to your game that the building was really heavily guarded. Is that 'railroading'? I don't think anyone would consider that to be the case.
Sorry, but I want to say "maybe". I need to know a bit more about the example and the context of play. For example, if the whole focus of the game up to this point has been about rescuing something from the building - the princess, the ransom money, the heirloom, the keys to the kingdom, whatever - and from the players' point of view
now is the natural point at which this should culminate, then if the GM sets up the situation such that the building is (relative to the mechanical capabilities of the PCs in the game system) impregnable, in effect the GM is railroading the players into missing out on their climax.
Generalising this point - I think that the excuse of "it's not the GM nudging or pushing the players, it's just the GM building the world" is never going to wash if the world that the GM is building is one that thwarts the purposes of the players in their play. (Note that this is obviously very different from thwarting the goals of the PCs - it's very common for the players to want their PCs to be challenged, and to face the risk of being thwarted - in 4e terms, this is what the DC and encounter building guidelines are for.) If the players want to play an exploration game, then it probably serves their purposes to let the GM build the world as s/he sees fit (provided it's not just a crappy or boring world), and in this case then the heavily guarded building probably wouldn't be a railroad. That's why context matters, I think.
I have never experienced a gaming group that used the shared authorship that you linked to. Not to sound too haughty, but it sounds like a bunch of DM-wannabes (or a DM who has problems being in authority). I could see such a group easily having problems because of shared authorship conflicts.
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One aspect of the gaming community "entitlement" movement is that it trains players that most anything is ok. Any idea is fair game, any PC build is fair game, any stunt is fair game, any feat is fair game. Everything is balanced and ok, and don't worry, the DM will adjust to any minor issues.
I think that there needs to be a fairly clear understanding as to who has what sort of control over what. (Of course, among friends at a relaxed table this understanding can emerge implicitly and organically.)
A few examples from my 4e game: One of the players wrote into his PC's backstory that the PC came from a city - Entekash - that had been sacked and destroyed by orcs and goblins. (This was in response to a direction from me, at the start of the campaign, that every PC had to have a reason to be ready to fight goblins.) This introduced a new city into the gameworld that I hadn't placed there. From my point of view fine, not a problem, I only had a small and local map of the starting area of the campaign, and having that player come from a ruined city was going to feed nicely into a whole lot of 4e thematic stuff like the role of Erathis (goddess of civilisation), the fall of Nerath etc.
A few sessions into the campaign the same PC died in a difficult encounter. I asked the player whether he wanted to keep playing the same PC, or a new one - and he wanted to stick with the same PC. It was already established that the PC was a disciple of the Raven Queen, so it was easy for me to narrate a meeting between the dead PC's spirit and his goddess. But I wanted to know
why the Raven Queen would send him back into the world. The encounter in question was happening in Nerathi ruins, with a symbol of Erathis inscribed on one wall. The player suggested that Erathis wanted the PC to come back and recover some sort of artefact hidden beneath the ruins. I agreed with that, and that's what happened (in mechanical terms, the PC came back with Raise Dead penalties, and the cost of a Raise Dead scroll was deducted by me from the next treasure parcel). The artefact was described by the player as a rod that would lead somewhere important by following the path of the old Nerathi roads, and we playd with that idea for a while. Subsequently, after DMG2 came out, I decided that it was also the first part of the Rod of Seven Parts, and this - and the PC's relationship to Erathis more generally - has gone on to be a significant part of the campaign.
In the same encounter, an NPC mage who had been taken prisoner by the party's paladin of the Raven Queen and coerced into coming along with the PCs and helping them was killed. It was already established that the encounter area, as well as being a Nerathi ruin, was infused with necrotic/undead energy. I therefore decided that, at a dramatic moment in the encounter, the dead mage reanimated as a wight and attacked the paladin. The player of the paladin asked if he could speak a prayer to the Raven Queen - I let him make a Religion roll against a Moderate DC as a minor action, and when he succeeded gave him combat advantage against the wight for a round. (Had he failed, I would have inflicted an appropriate amount of damage on his PC as necrotic backlash.) It was clear both to the player and to me that part of the context for this player-initiated action was the prior relationship that had been established, via play, between the NPC and the PC.
A final example. In another encounter in which the PCs were sitting with some elves around the campfire, the player of the renegade drow - who was already established as a Corellon worshipper - explained to me that he was part of a secret society that had members among the elves of the surface world, and that he was making a secret hand signal to see if the captain of these elves responded. I replied that the captain did not, but that the elven crafter did. This later became relevant when the PC in question wanted a dragon tooth made into a wyrmtooth dagger, and has fed into the campaign in other ways.
These are all examples of player authorship in one way or another, but in each case they advanced the game. Part of what works in these cases is that (i) the player's suggested addition to the game fits into the prior established story of that player's PC, and (ii) it's understood by everyone at the table that I, as GM, get to frame the situation, but am generally happy to incorporate player ideas that will enrich it or make it more complex and engaging. The players aren't entitled just to author a solution that difuses the current situation, but my players generally don't try and do that.