I GMed a session of In A Wicked Age for a couple of teenagers a week or so ago.Let's take the extreme (and ridiculous example) of the player whipping out his helicopter to travel from A to B. Now, let's be honest here, that's not something the other players are going to be groovy with, so, it's perfectly understandable that the DM steps in. But, let's stipulate that the players all think this is a great idea and it's tons of fun. At that point, who is being served when the DM says, "Nope, no helicopters"? It's making the DM happy and making everyone else unhappy.
One of those players was playing the Warlord Romulus. Whose pet cat was Mr Fluffington. While my image of the setting was pulp-y, swords & sorcery West/Central Asia (along the lines of REH, and reinforced by the default name lists), the other player described their illusionist PC as being dressed in the style of a court jester - and held up the joker from a deck of cards to illustrate.
If I had sole authorship of the fiction, those particular story element would not have turned up. But there seemed to be no need to contradict what the players were introducing. It's their fiction too!
A different example, also inspired by the helicopter, fits with @Campbell's idea of the system taking some load of the GM. Here's the story:
What made this particularly easy is that 4e has a robust system for assessing the value of treasure, and the amount of treasure to be awarded per PC level. So the Perception check didn't determine whether or not the players get a free bennie, but only whether or not they get it here and now when they want it, or - if it had failed, and I'd narrated some appropriate complication - whether something else comes to pass before they get it.The PCs in my 4e game have gone to the Feywild looking for the Glacial Rift of the Frost Giant Jarl, so they can destroy the Frost Giants who are massing, in alliance with Lolth and the Prince of Frost, to start a War of Seasons that will overthrow the Summer Fey and steal control over winter away from the Raven Queen.
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I'm using photocopies of my old G2 maps, blown up onto A3 paper, with notes on cavern occupants and stats written up where necessary.
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At the end of the session, the eladrin, edritch giant and griffons were all dead, and the invoker had tamed the giant's frosthawk (with a successful Nature check plus some gentle words spoken in elven). The paladin was inside the remorhaz (but, being a tiefling, was mostly enduring the auto-fire damage it does to swallowed creatures).
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The next session began with the ranger spotting the 5th PC, the dwarf fighter wielding Overwhelm, in melee with a group of giants on the ledge at the other end of the rift (around areas 16-20, for those who know the module).
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Eventually the sorcerer killed the remorhaz and the paladin was able to cut himself out, but they then had to make it to the other end of the rift. The sorcerer has at-will fly (via Dominant Winds) but that is not very good for two people.
Then a solution suggested itself.
In an earlier session (linked to above), the PCs had helped an eladrin noble deal with a demon that was cursing his apple grove. I told the players that the noble gave them a reward, and gave them licence to choose their own item or items of 28th level or equivalent value. They chose some sensible, eladrin-noble-appropriate stuff (a couple of elfin chain shirts, the winged boots, a ring of regeneration and a surge-boosting belt) but the player of the sorcerer also liked the idea of the 25th level magical vehicle the Thundercloud Tower (from a Dungeon magazine, maybe one of the Giants ones). It seemed unlikely that an eladrin noble had such a thing on-hand to gift to them, so we agreed that the best they got was to learn rumour of its existence on the Elemental Chaos from the noble, while discussing the threat that the Elemental Chaos (especially its giants) poses to the Feywild.
It had already been discussed that the Glacial Rift was very cold (the PCs are under the protection of an Endure Primordial Elements ritual, cast by the sorcerer), infused with the stuff of the Elemental Chaos. And so the player of the sorcerer decided that perhaps the Thundercloud Tower was somewhere here, having crossed over from the Elemental Chaos. This actually wasn't as farfetched as it might seem, because I had already decided that the mad Storm Giant Mirkamaur (sp?), a servant of the Crushing Wave detailed in the Plane Below, was visiting the giants (in the original I think it is a storm giant princess who is in the lower levels), and a Thundercloud Tower seemed like the sort of vehicle that he might travel in.
So the player made a perception check, assisted by the player of the paladin, and indeed they realised that one of the spires of rock half-buried in snow and wind-blown ice was in fact not a natural outcropping at all, but a 30' tall tower. They made their way in, up the stairs and to the top where the drow made an Arcana check to attune himself to the control circle for the tower. The next round they were up and away.
I think everything that a GM, and a system, can do to help make it easy to say "yes", and/or to frame checks so that the players have their chance to impose their conception onto the fiction as an upshot of succeeding, is making life easier for the GM, and helping ensure a cohesive and enjoyable shared fiction.
Whereas I think a lot of "gating", postponement, saying "not", etc tends to have the opposite effect. It makes the players turtle, disengage, even become resentful. And to what end?