I think of it as a sub-genre within which one can play old-school D&D. If you imagine that Ravenloft is "D&D as gothic horror" and Dark Sun is "D&D as post-apocalyptic" then Fantasy Fucking Vietnam is "D&D as a Vietnam war movie".
I consider the styles inspirations to be movies like Apocalypse Now, Deer Hunter, Full Metal Jacket, and Platoon, and books like Glen Cook's The Black Company and Joe Abercrombie's The Heroes.
Here's what it's like in play:
1. PC casualties are high. Death is random and sometimes pointless. Moral virtue is no insulation.
2. The PCs might have the equivalent of tanks (fighters in magic plate armor) and air support (mages with fireballs) but the NPCs are dangerous because they have viciousness and cunning. They use poisoned punji stakes, hidden ambushes, and twisting underground tunnels in the wilderness.
3. It's not particularly clear that the PCs are good or that their cause is just. But it's not that the NPCs are good or just either; in fact, the NPCs are inhumane and ruthless. Both sides do awful things. The innocent bystanders tend to get killed.
4. Nobody goes into the Shit and comes out unchanged. Characters get cursed, mutated, polymorphed, their Constitution gets depleted, their levels get drained. There are fates worse than death.
5. By the time adventurers are successful enough to be seen as heroes or lords, they are too damaged to enjoy the fame and glory.
6. In its bleakness, it has similarities to Lovecraftian horror, except that the soul-destroying evils don't require Cthulhu; they are inflicted by man on man.
This is more-or-less my favored GMing style. The permanent wound chart and tampering with mortality chart in ACKS give some of the overtones.