Although Mieville says he hasn't played an RPG in years, and is unlikely to play again, he still enjoys reading RPG supplements - especially bestiaries. Not only this, but Mieville was even a guest author on the 2010 Pathfinder supplement Guide to the River Kingdoms. In terms of D&D's influence on Mieville's writing, the theme that keeps popping up in interviews (see below) is systemization. Mieville cites the Monster Manual as being a huge influence because of the way it catalogs and numerically describes horrifying and mysterious creatures. Indeed, one of the things I've noticed about Mieville's writing is that once you crack the surface weirdness - all the fabulous settings and creatures - his worlds are largely mundane and operate in a very systematic and naturalistic fashion. Like Gygaxian D&D, you can get a grasp on how things work and "learn the rules", and those rules don't change. Ironically, this is one of the things I like least about the Mieville I've read (i.e. Perdido Street Station). One of my favorite buzzes from fantastic fiction is the creepy disorientation resulting from small events that imply major and possibly horrific problems with the state of natural world as previously understood - namely, when you think you know the rules, but then realize you don't really. Mieville's fiction is creative and fun and I enjoy it, but I definitely don't get this kind of ecstatic Lovecraftian vibe from it. Maybe this literary preference of mine is related to why I almost never use the Monster Manual for my games - I love unique monsters and the dampening effect they have on metagaming... and systemization.
Mieville is much more eloquent and thoughtful than me, so I'll let him speak for himself. Here are some excerpts from two Mieville interviews that deal with RPGs specifically. A third highly relevant interview, for which there is no web link, was published in Dragon Magazine #352 (along with an amazing 40-page RPG treatment of Mieville's world Bas-Lag). If there's reader interest I could post something about this issue of Dragon...
A 2005 interview from Believer Magazine:
BLVR: Judah Low is a golemist. Golems are only ever clay. The only place I’m aware of them being comprised of other substances is in the old AD&D Monster Manual. The original Monster Manual. They had a picture of a flesh golem with bolts in his neck—
CM: I know the picture very well.
BLVR: It’s Mary Shelley’s Frankenstein. That implied subtext—that Frankenstein was a type of golem—made my pre-teen mind vibrate with a promise of special insight.
CM: You are absolutely right. I use AD&D-type fascination with teratology in a lot of my books, and I have the original Monster Manual, and the Monster Manual 2, and the Fiend Folio. I still collect role-playing game bestiaries, because I find that kind of fascination with the creation of the monstrous tremendously inspiring, basically. And the golem that you’re talking about in AD&D, it’s very perspicacious of you, because that is directly an influence. There is a scene in Iron Council where Judah creates a golem of corpses. He shoves his hand into a pile of corpses and makes them into this huge, lumbering golem of dead people, and that is a riff on the flesh golem. One of the things that I love so much about fantasy and science fiction is that the weirdness that it creates is always at its best completely its own end and also metaphorically and symbolically laden. I get very frustrated when I read certain types of magical realism and you end up saying, “Okay, I understand this figure of this golden elf is symbolizing such and such.” The thing about genre fantasy is that it is its own end, but it also does that job of symbolizing. I think about something like Gulliver’s Travels. The figures of the Lilliputians are partly a way for Gulliver to overlook society from a godlike height and to make satirical, symbolic comments, but it’s also, “Hey look, little tiny people! How cool!” I love the idea of golems. It strikes me as a very powerful, imaginative, weird idea. But it’s also an idea that is symbolically fraught and laden, and particularly in a book which is partially about people, politically speaking, people taking control of the fruits of their own labor. So, the golems are both just really cool monsters but also something that functions as part of the political texture of the book.
A 2003 interview with Joan Gordon:
JG: What cultural influences shaped your writing?
Probably one of the most enduring influences on me was a childhood playing RPGs: Dungeons and Dragons [D&D] and others. I’ve not played for sixteen years and have absolutely no intention of starting again, but I still buy and read the manuals occasionally. There were two things about them that particularly influenced me. One was the mania for cataloguing the fantastic: if you play them for any length of time, you get to know pretty much all the mythological beasts of all pantheons out there, along with a fair bit of the theology. I still love all that—I collect fantastic bestiaries, and one of the main spurs to write a secondary-world fantasy was to invent a bunch of monsters, half of which I’m sure I’ll never be able to fit into any books.
The other, more nebulous, but very strong influence of RPGs was the weird fetish for systematization, the way everything is reduced to “game stats.” If you take something like Cthulhu in Lovecraft, for example, it is completely incomprehensible and beyond all human categorization. But in the game Call of Cthulhu, you see Cthulhu’s “strength,” “dexterity,” and so on, carefully expressed numerically. There’s something superheroically banalifying about that approach to the fantastic. On one level it misses the point entirely, but I must admit it appeals to me in its application of some weirdly misplaced rigor onto the fantastic: it’s a kind of exaggeratedly precise approach to secondary world creation.
I’m conscious of the problems with that: probably my favorite piece of fantastic-world creation ever is the VIRICONIUM series by M. John Harrison [The Pastel City (1971), A Storm of Wings (1980), In Viriconium (1982), and Viriconium Nights (1984; rev. 1985)], which is carefully constructed to avoid any domestication, and which thereby brilliantly achieves the kind of alienating atmosphere I’m constantly striving for, so it’s not as if I think that quantification is the “correct” way to construct a world. But it’s one that appeals to the anal kid in me. To that extent, though I wouldn’t compare myself to Harrison in terms of quality, I sometimes feel as if, formally, my stuff is a cross between Viriconium and D&D.
JG: You mentioned being drawn to the systematization in RPGs. How do you see that in your writing?
CM: I start with maps, histories, time lines, things like that. I spend a lot of time working on stuff that may or may not actually find its way into the novel, but I know a lot more about the world than makes it into the stories. That’s the “RPG” factor: it’s about systematizing the world.
But though that’s my method, I don’t start with it. I don’t start with a bunch of graph papers and rulers. When I’m writing a book, generally I start with the mood and setting, along with a couple of specific images—things that have come into my head, totally abstracted from any narrative, that I’ve fixated on. After that, I construct a world, or an area, into which that general setting, that atmosphere, and the specific images I’ve focused on can fit. It’s at that stage that the systematization begins for me.
I hope this doesn’t sound pompous, but that’s how I see the best weird fiction as the intersection of the traditions of Surrealism with those of pulp. I don’t start with the graph paper and the calculators like a particular kind of D&D dungeonmaster: I start with an image, as unreal and affecting as possible, just like the Surrealists. But then I systematize it, and move into a different kind of tradition.