There won't be any spoilers here, I'll just be discussing A Game of Thrones in general terms. The stories have a number of engaging elements; they follow the rise to fame and infamy of the scions of various noble houses in the fictional land of Westeros during a civil war. The king dies in the first book with a contested heir and the novels relate the chaos in the aftermath. No one has any plot immunity. Life in Westeros can be nasty, brutish and short, and main characters frequently get offed. From that perspective, there is always anticipation of 'what will happen next' when reading GRR Martin.
Westeros itself is very familiar. The map always seemed to me a fantastic version of England, stretched to the size of a continent. Hadrian's Wall is replaced by the gigantic ice wall, separating the settled areas from the wild north. Folks have commented that the war between the Starks and Lannisters is reminiscent of the war between the Yorks and Lancasters; the names share more than a similar phonetic cadence.
The fantastic elements in Westeros are subdued so as not to overwhelm the human element of the conflicts. There are hints of The Others, ice-born undead beyond the great wall, and legends of a time of dragons, but most of the main characters are disinterested in magic or stories. Certainly these fantastic elements have gotten more prevalent as the series has progressed.
How would Westeros work as a D&D setting? The archetypical career of the D&D character, as an explorer and dungeon delver, doesn't seem to have a place in the literary version of the setting. There are scant mentions of ruins, and there's certainly no freeman profession of "professional adventurer" the way we see it in more typical D&D worlds. The literary version wouldn't seem to support wealth through salvage. Mercenaries, yes, but not gold-claiming monster-slayers. Characters in A Game of Thrones rise to power through politics and feats of arms. Wifey was telling me - Oh c'mon, you could put old ruins beyond the Ice Wall, or beneath Harrenhall Castle, or on one of the many islands.
Why not just use a standard D&D setting and add a layer of politics and sheer bloody mindedness? Chances are your D&D setting already has some rulers who were previously adventurers, and a D&D setting assumes it's common for self-made rulers to rise to prominence after starting their careers as dungeon-sacking adventurers.
However, I can think of some things to strip out of D&D to make it fit the theme. I'd de-emphasize alignment. The question of right and wrong is highly subjective in A Game of Thrones, and a sympathetic case can be made for each side. The vast majority of monsters need to be removed as a force in the world. It's fine if you have some isolated monsters in the ruins or on the frontier, but there are no apparent global monster threats that would trigger the human kingdoms to unite; there's nothing to distract the nobles from beating on each other. Many D&D worlds follow the Tolkien model of the big bad evil guy, providing a rallying cry for all the 'free peoples' to band together for all mankind and sing the campfire songs. Not in Westeros.
I'm ambivalent about the degree of magic. Westeros is seemingly low magic; there are few flashy spells or planar entities or magic weapons getting swung around. The gods seem distant and remote, but there is evidence of magic-working priests and priestesses. There's even a fair amount of 'raise dead' that happens in the stories, although the results aren't necessarily comfortable or desirable… more like revenants.
I've been using a list of campaign criteria for a good D&D setting to help the analysis, here's how it shakes out vis a vis Westeros:
Adventures and Frontiers, Autonomy, Dungeons, XP for Gold, Treasure and Magic Items, Classes and Levels, NPC Classes and Levels, Alignment, High Magic, Humanoids and Monsters, The End Game, Demi-Humans, Clerical Magic, Vancian Magic
We've already said the setting would need to be altered by adding the ruins of prior civilizations to support an adventurer class, autonomous explorers, XP for gold, recovery of treasure and lost magic, and so on. (Valyrian steel!). Plenty of the rulers and main characters appear to be tough, high level characters, in game terms, so there's no problem there, and the standard D&D end-game of rising to power and claiming a domain works very well here. Alignment needs to be removed or face an adjustment, and humanoids and monsters must be marginalized (same for demi-humans). The political conflicts are human vs human, there's no joining hands with the friendly elves to fight Sauron or Iuz or Tzass Tamm. I'm ambivalent about low versus high magic, but tend to think low magic would work better in terms of flavor, whereas high magic would require less tweaking of the game engine.
What do you think? Have you tried playing a D&D game in Westeros, or borrowing ideas and themes from A Game of Thrones into your game?