This goes back to my own rule of thumb on maneuvers.  Without a LOT of good reasons, I do not allow aspects from maneuvers that can not, in turn, be countered/removed by another maneuver of some form.  "Beheaded" or "Disintegrated" certainly fall into that area.  I leave those sort of permanent, life altering aspects to come from consequences.

This.  Why even have a conflict system if maneuvers can take people out?  And its worth noting that even consequences can't be life ending, so "beheaded" can't be a consequence (for most targets at least).
Otherwise you (or thew GM) could just create a social powerhouse character and maneuver for the aspect of "willing to do whatever I say" against anybody who comes along.  Not much of a game then, eh?

Another rule of thumb is that if a manuever places an aspect that's gonna last, the person who placed that aspect has to stick around to make sure it stays in effect.  Thus, you can try a maneuver to place the aspect of "grabbed by the neck" on somebody (and keep it in effect by hanging on) but not "beheaded" (since that would last without any effort from you).

Well, if all yer talking about is using a long-arm to block / parry with, I can see that as a defense against fists or melee, sure.  That's pretty much just the weapons skill, used with a (rather expensive) club.  But how does that protect against magic, any more than having melee and a sword would?

I dunno, Gun Kata seems a bit to good.  It seems like it potentially lets you avoid buying a range of skills that over-all are already less combat effective than Guns.  Can any other single combat (or other mundane) skill defend against all physical attacks (Fist, Weapon, Gun or Magic), assuming you have a 1 refresh stunt?  If not (or even so), without costing something for each use it seems to easy.  Requires a fate point?  Counts as your next action?  Uses up all your ammo?  Seems like there should be SOME cost per use besides just the refresh.

So... the point of this is as such: who has games with wiseasses and who doesn't?  And for those of us whose groups are a little more serious, does it affect the feel of the world tremendously?

In our game, I don't think it's so much "wise ass" as "genre savvy".  For example, when its time for my "true kung fu" character (who can call up various abilities as martial arts styles, via a special implementation of Mutable Power) to make a fists roll, I try to always call out some cinematic / anime style kung fu manuever name.  Last night I jumped on some red court vamp, wrapped my legs around his arms, and started pounding his head; that was "Monkey Eating Watermelon" (I was using Monkey Style, which given the setups I took, seems the most powerful style I have).  When he tried to hit me with a free arm, I blocked with "ducks around tree defense".  Eventually he got thrown flat (by the magicians spell, aimed to place an aspect) and I pulled out a vial of holy water to use as like brass knuckles- landing the "Tears of Father Damion Falcon Punch" for a whopping net 9 stress.  (I figure if my character is made mostly to fight, I'd damn well better put on a good show when doing so.)
Of course, there's always the obligatory "I really don't want to be doing this" type statements - made as you are obviously fully intending to do whatever it is will bring trouble (and heroism, and fate points) on your head.

(Note- this Martial Artist is the same one Dian's Victorian scholar was talking to.  So yeah, hopefully the mage and I can riff a bit- sadly, my inept comments about magic is not nearly as amusing.  It seems my little knowledge is enough to be dangerous, but not quite enough to be funny.)

I'll add a few fight-related physicals I figure to use for my martial artist.  I tried to make them easy to compel rather than anatomically descriptive

Mild- fat lip, sore, sprained, winded, tired, banged up, looks worse than it really is, wincing

Moderate- tasting blood, can't wear that any more, shouldn't be walking on that, making a mess, ragged breathing, don't touch me there, partly blinded, partly deafened, cussing

Severe- picking up teeth, can't walk on that, holding it shut, gasping for air, punch drunk, tunnel vision, moaning agony

There's very few ways for a normal human to disable (and silence) another normal human non-lethally, yes.  But for supernatural characters, you could have all sorts of things; stingers with knock out toxin, vulcan nerve pinches, kung-fu chi strikes, hypnotic gaze, etc.  Many Dresnden characters who aren't mortals would likely be able to find a way to do it. 

I'd initially tried to do it without mortal stunts, but you really can't capture the "martial arts movie" flavor without them, and I would have ended up resorting to superhuman levels of power (which I didn't really want) just for variety between styles.  I also didn't want the stuff that a martial arts movie character couldn't have, like claws or wings, so again was pretty stuck for variety.  Modular Ability is obviously made with something rather different in mind- I don't think its meant for people who manifest new powers due to activating enchantments / focusing chi / etc.

And yeah, if Modular Ability could be used to access to an unlimited number of any sort of mortal stunts, it would be pretty imbalanced.  Hopefully having it give access to a limited number of thematic mortal stunts is more or less on par with the effects you could get from things a wizard can do (in this case by using various effects to boost his own physical prowess).  That's what I was aiming for.

If I knew the system a bit better, I might have gone for an actual wizard, one who used various effects to achieve what is normally described as "martial arts"; I'm not sure if the system entirely allows that, though, and I expect the flavor would be rather different.