Fred Hicks says:
Here's what I'd do to make Fate hurt more.
Reduce the number of stress boxes
The simplest change. Give everyone a base of 2 or 3 instead of 5, and those fights will start feeling painful a lot faster. Making the base 2, you'll encounter a setup where 5 stress boxes — the usual default for SOTC — is what *tough* people get. Should feel a bit more right.
Change how stress boxes behave
Make 'em act like minion stress boxes (hit points, essentially), where if you take a 3-stress hit, you mark off 3 boxes. Players may be motivated to walk the consequence line mild-moderate-severe whenever a big hit comes in, and let their stress track soak up the smaller hits by degrees.
Change how consequences work.
Make the stress track work like this: when you exceed your stress boxes, you're taken out. Period.
Consequences let you do this: reduce the amount of stress you take from a particular blow. Whatever's left over would get marked in the appropriate box on your stress track.
Take a mild consequence for a -2 to the stress dealt.
Take a moderate consequence for a -4 to the stress dealt.
Take a severe consequence for a -6 to the stress dealt.
I find this makes fights pretty interesting. Suppose you have a 5-box stress track. Someone nails you massively, a 8 point hit. Taking a mild consequence isn't going to do it — that leaves you with 6 stress, still beyond your stress track. You have to take a moderate one, at least, to make it work, reducing the stress down to 4, and thankfully landing inside your stress track. You'd then mark off the 4 box. The nice thing about this is it breaks up the predictability of the mild->moderate->severe consequence path.
Keep in mind, too, that if you use any stunts that force a consequence on the target, it doesn't come with any of the normal benefits. So, if I hit you for 4 stress and spend a fate point to use some stunt to inflict a consequence as well, you're still taking the 4 stress… you have to use another consequence to absorb the damage.
See also Varying Consequences by Weapon for a mod of this option.
Combine 2 or more of the above methods
You want super lethal? Combine all three. A base of 2, acting like hitpoints, with stress reducing consequences and a "you run out of stress, you're taken out" philosophy will keep things smooth and fast but will harken back to the terrifying lethality of your Rolemaster days.
Take any two of the above and you're still gonna see a lot of blood hit the pavement.