Who needs therapy if you've got "impactful" RPGs, right?
Impactful is such a weird word. I've had lots of memorable sessions. Impactful seems like "something that changed you outside the game" and I'm not convinced that's why I play RPGs, so…
Man, I dunno.
Okay, here's one, maybe.
Dogs in the Vineyard. Our second serious fight. We'd already had one smaller altercation earlier in the game, so the players had seen how the conflicts worked, and more to the point they saw how FALLOUT worked.
Second conflict, things started getting heated, and someone pulled out a gun.
And the players – some experienced, pretty seen-it-all gamers – kind of pulled back from the table and were like "Whoa. Shit. Hang on a sec."
That game made guns fucking scary. They made them as goddamn dangerous as they are, you know?
(Best of all, the way fallout worked, you got fights where you didn't really know how fucked you were until the shooting stopped.)
Dogs remains the only game in which the players (and, by extension, the characters) reacted to someone pulling out a gun the way real people would, and I (obviously) remember that, to this day.
I always strive to make violence pause-worthy in my games. Dogs does a good job of both pushing people in that direction to get what they want AND making them cringe when they get it.
Oh, yes. Guns, or, in fact, any kind of violence gets way out of hand, very quickly, with dire consequences, in that system/setting.
I'd love to see something like that impact extended into other "real life" game systems. Because, really, most real people react very strongly (and tend to freeze up) when fists, knives, or guns get brought into play.
Honestly, the game system itself is quite hackable. It would be pretty easy to use it in lots of settings.
There are a couple character and background templates to give you a set of skills and experiences with die-sizes attached (basically split between Stuff that Goes Well for You and Stuff that Causes you Trouble), and then you build some gear by attaching descriptors to it that determine it's die size for conflicts.
(It's the size of the dice in play that determine the severity of Fallout. Guns in DitV were the worst, at d10. Knives and quite a few ritual items and stuff were d8s. Et cetera.)
The d8 stuff was hard-but-not-impossible to get a fatal result. I seem to recall one bad guy needing to roll something like 16d8 of fallout after one fight, which pretty much guaranteed he'd die.
There's a sort of… Wizard of Earthsea/Tamora Pierce flavored spin-off of DitV that capped dice at d8s, which left scary violence on the table, but not QUITE at the level of the original. It worked well.
As much as I would like to hack DitV, the setting so completely got under my skin. I don't think I could if I wanted.