I see killer blow as being a case of "if you really absolutely must wreck complete devastation" kind of thing.  Sure, its more economical to just tag two
aspects in most circumstances, but killer blow can be used in conjunction with regular aspect tagging to take damage beyond the potential of a guy who can
just tag aspects.

My point was that it does not particularly "take damage beyond" that potential: between two otherwise equivalent opponents, the guy with the stunt, Killer, has one less fate point than the guy without, called Fred. Thus Fred can tag two aspects on that first hit and get +4 whereas Killer can only spend one fate point, but gets +3 for it. At the second exchange they are even. At the third exchange, Killer pulls ahead by one point of damage, but by this time they've both spent 5 fate point in three exchanges. I'd expect a little more zing with such a deep investment before it pays off.  (And again, I haven't play-tested it, so maybe in practice Killer would spend 5 or 6 fate points a session on his Blow and it's worth it).

But I like void's point, so I'll take a look at the other Fists stunts:

• Footwork: looks great. It makes me want to name it "Float like a butterfly" and have a corresponding "Sting like a bee" stunt.
• Martial arts: I had assumed that assessments and declarations could be made from any skill, in which case this stunt would be weak. If that otherwise must be a specific trapping, then this looks good.
• Armed arts: looks great. I was initially comparing this to a SotC skill-substitution stunt, but I like that those were toned down a fair bit. This has nice color
• Lethal Weapon: the restriction to unarmored opponents seems unnecessary and limiting. It substantially drops the effectiveness of martial artists in
  many common scenarios, for example magical defence can be used as armor (and will be more frequently with the new crafting rules). Just giving the damage bonus reflects things like your bones really are thicker/harder because they been subjected to the relevant stress, you do hit harder because you don't subconciously flinch, etc.
• Redirected Force: looks great. It seems much weaker than Riposte though. See below
• Step into the blow: looks great. It is an interesting complement to Redirected Force; together, if your defense succeeds, you place an aspect. If it fails, you hit them. Maybe it's the one I will call "Stings Like a Bee"

Riposte seems too powerful: as described, it precludes the attacker from employing other means of defense (e.g., it let's and axe wielder parry the mage's wimpy athame attack, skip right past his +10 magical block, do potentially wopping weapon damage, and potentially pop the shield since it was bypassed to boot). Prototypical fencing also has people countering a riposte, setting up an opportunity for it to lead people in, etc. With this stunt, the target cannot even invoke aspects to save himself form the attack. Perhaps permits you an immediate attack at a bonus or some free stress (as in SotC), but not obviate the combat process and all the mechanisms that it engages. Since ripostes worth mentioning are interesting storywise, perhaps it allows you to spend a fate point for a free immediate attack (possibly even at a minus; it is free, after all).

I don't understand the logic of requiring Martial Arts for any of these stunts though. They already are mortal stunts, which in theory means they can have a little more oomph. With lethal weapon, you hit could hit them like a pile driver whether they are doing karate or the Texas Two-step. Perhaps a motivating reason is that without this incentive, people might not buy the declarations/assessment trapping, and thus miss out on a lot of fun/storypower.

FWIW, Demoralizing Stance has been great fun in SotC play.

And again, all this is based on analysis rather than playtest, so take it with a grain of salt as "just trying to help". Overall, I'm just absolutely delighted with the rules .