Game Mechanics that set the tone

So I’m musing about game mechanics; have been for the last couple weeks, actually, because I’m playing a lot of Spirit of the Century, editing a MONSTER of an old-school-style game called Robots and Rapiers, getting ready to run Galactic, and wishing I’d had more luck playing PTA and Dogs.
So all those systems are bouncing around in my head, and I start drawing comparisons.
Here’s an observation:
Lots of games have Edges. By “edge” I just mean “that thing on your character sheet that lets you tweak things in your favor.” Call them Aspects, Traits, Talents, whatever… in play, they let you tweak results.
There are really two ways that an “edge” can be invoked:
Mode 1. They can be used to give you a intial, “pre-roll” boost to your chances of success, thereby increasing your odds of winning a conflict. Primetime Adventures does this with both it’s Traits and Fan Mail. Spirit of the Century pretty much does this with Aspects (they come in after the roll, but before the roll *counts*).
Mode 2. They can be used to stave off or lessen the sting of failure. Galactic’s “Edges” do this. Traits you bring in after a conflict has already started in Dogs in the Vineyard do this. “Doom” in Conspiracy of Shadows does this. The appropriately-named Survival Points in Dead of Night do this. There are many others.
Now, my point is this: your final numeric result using ‘edges’ in either of the two ways above might be exactly the same, but the modes feel different, and that feeling seeps into the tone of the game you’re running. pushing either toward adventure-heroic (mode 1) or the survivalist-gritty (mode 2).
I’m not talking about the game’s power level. It doesn’t matter if you’re giving folks one ‘edge’, or five, or ten — I think if they’re implemented in the style of Mode 1, the game is going to have a kind of “let’s be awesome” feel, and if you’re using them in the style of Mode 2, it’s going to have a kind of “let’s survive this” feel.
What does that mean? I think that means that, even if you have a mechanically-perfect ‘hack’ to the Spirit of the Century rules to use it for zombie-survival-horror, unless you change the way you can invoke Aspects, the system itself will be subtly encouraging you and the players ‘be awesome and heroic.’ It’s not the number of Aspects you give people that matters, but how they can be used that will affect the tone.
Now, let’s say that you have a group sitting at the table who (a) totally gets the tone you’re going for, (b) agrees to it and (c) actively works to support it. Can they overcome the subtle whispers of the game and run an horrifying zombie-survival game using, say, straight Spirit of the Century?
Yes. Without hesitation, yes. The rules are only one voice at the table, and can be drowned out easily but other voices. It’s really no different — or less jarring — than when one PLAYER is working toward a different tone than everyone else.
You just can’t throw popcorn at the rules and say “knock it off.”