"Who are your influences?"
"Animal in the Muppets."
- The Commitments
GURPS Dungeon Fantasy's influences are a mishmash of video games (Diablo 2, Rogue), old-school RPGs (D&D, AD&D, T&T, C&S, and more), and the circular influence of GURPS games run by the author(s). As is made clear in the designer's notes and here and here, too.
GURPS, straight out of the box, does serious, high-lethality fantasy very well. Everything from social standing to combat to mass combat is covered along the way, so it's fine for a wide-ranging sandbox. So much so it was the go-to fantasy game for me for decades before DF rolled along. DF turns it on its head and says, what if your guy starts out as an ass-kicker and we toss all the serious stuff about social engagement outside the dungeon aside and just concentrate on that? Throw in some deliberate goofiness sometimes and you get a beer-and-pretzels game. What Mike Mornard claims Gary Gygax said about his game - to paraphrase, it's just a stupid game.
This is partly why, for example, Dungeon Fantasy delvers are so powerful - it's a rare video game that forces you into what is now called a character funnel. Old ones like Wizardry did it, but from experience I can tell you it was grueling and not always much fun.
In any case, games like that influenced me in my writings for DF and in my GURPS games, "straight" fantasy and DF alike.
If I had to list my influences for my DF game, in no particular order, it's a mix of:
AD&D - especially my post-UA game
D&D (Basic set, merged with AD&D and Holmes and criticals from Arms Law)
Rolemaster, 2nd edition (the blue box)
GURPS (my own 1st and 3rd edition games, via recycled monsters and treasures)
Fighting Fantasy Gamebooks by that other Steve Jackson
Gritty fantasy fiction (Glen Cook, especially)
A myriad of video games (Diablo, D2, Wizardry, Bard's Tale, Wizard's Crown, Planescape: Torment, and more)
OD&D, in that I always look into the LBBs and their excellent supplements for crazy 40-year old craziness to just toss in.
My minis collection, since this is my chance to use all the monster minis I have.
Even some non-fantasy games have a strong influence on this game: a little of Jade Empire, the attitude of 1st and 2nd edition Gamma World, and my own brutally character-grinding GURPS version of the Yaquinto Pirates & Plunder starting adventure.
The summaries of the Dwimmermount campaign helped push me into going for a megadungeon instead of a series of smaller dungeons, too.
To some extent swords and sorcery and Vance influence me now, but their influence came fairly late - I got introduced to Moorcock, Vance, Leiber, and Howard through D&D, not the other way around. They inform my current game to varying degrees, though - my previous game showed a lot more Leiber and Vance and Moorcock than this one, but there is still a bit that shows through.
So, who are your influences? What is informing your decisions about what is cool and necessary in your game? What really shows up in your day to day play?