I often speak of defensive fixed blade knives to be carried offhand. Dan at BladeReviews mentioned that he appreciated my input on such things so I'm going to over-clarify. These are my opinions and observations, but of course, I've never actually gone at anyone with a blade. If you think it's crap, please comment away!
First off one must remember that a defensive knife isn't the same thing as a true dedicated fighting knife, nor is it going to be a combat knife. Fighting knives have their own special requirements. The foremost is the ability to deliver an immediate kill. Therefore, the fighting blade must be either deliver catastrophic damage through edge cuts or internal organ hits through point penetration. Combat knives for me, are knives that are primarily field tools that also can do a fair job as a fighting blade when necessary.
For an edge kill, that means a large knife with the weight and design physics to chop or slice through bone and muscle. Not just taking out arteries. Death by blood loss takes time. Time that can be used against you. In addition to the edge cut there is also the blunt force issues of bone and meat being pushed into other organs or places. Shock, the shear level of physical damage, and massive blood loss all together would finish off the victim. Kukris, barongs, large bowies, and such are renown for this sort of thing.
You'll find that penetration kills are more common. It is easier and falls in line of the fighting blade's role in the modern military world. Silent kills on enemy soldiers by special forces operators. These are ambush kills, but kills nonetheless. Here, length is the big issue. Like with the minimum penetration standards the FBI put out for handgun projectiles, there is a literal distance to be traveled to the life sustaining organs of the human body from the point of entry. If the blade is strong enough, it can penetrate the rib cage directly. The cage can be bypassed by going up under it or down through the collar. If one looks at the bayonets of the world you can figure out the minimum size for such weapons. The modern bayonet seems to be in the range of 6.5 to 8 inches in length. That is a reasonable bar then. Also, the blades tend not to be broad and are either double edged or the back is swedged to give better penetration.
But what does all this tell us about the defensive knife? Merely, what we cannot expect from it. Defensive blades are not meant to deliver a killing stroke. They can cause fatal wounds, but very rarely an immediate one. Look at a cat's claws for a great example. Cat's do not kill often with their claws, especially not something close to it's size. In predatory action, the claws hold while the bite kills. In a fight, the cat often curls up and uses its powerful rear legs to shred the soft places of its opponent. As we cat owners know, this crap hurts, even when they're just playing. The cat scratches you to make you let go. That is of course the end game. Getting free.
Your defensive blade is meant to inflict as much hurt as it can at point blank contact distance to make the goon let you go. As a bonus, it might seriously weaken that same person. Every drop of blood the other guy loses is helping the fight go in your favor. Also, like with a cat, he should never have seen that blade until he feels the tip scraping across his bones. Shock is useful! Fear and surprise can help you. If you can change his mind from fight to flight, it is your win.
Defensive blades, naturally have some serious requirements. For the blade, while length isn't as important, it must be kept deadly sharp. Just like you should keep a kitchen knife. After all, you're cutting meat. If the knife is too long it will not be easy to bring into use against a grappling attacker. The steel doesn't have to be uber premium. AUS8 and 440c do well for blades of 4 inch and under. This also has the benefit of not costing as much to replace if the knife must be left embedded in someone. Also, you want to be able to draw the knife and use it.
The grip needs to be excellent for both draw and retention. Ever try to pull a smooth gripped knife out of a kydex sheath? Or keep hold of a slimy grip? I find G10 grips to be wonderfully rough and grippy in the worst situations, but most poly, wood, or metal grips with good texturing will do nicely. Also, sub hilts and ring hilts help a lot in both draw and retention. They lock your finger in, though if the blade is caught, you might get something broken. Also, a good guard is a must too. I think too many knives do not have adequate guards. They're really not for protecting against other knives, but protecting you from a hand-slip and some cut fingers.
Perhaps the most neglected issue from the knife manufactures is carry options for their defensive blades. Most knife manufactures do not provide even decent sheaths with their knives. Many folks are forced to buy 3rd party sheath systems or make their own. IWB, OWB, pocket, neck, and even should carry are possible, but with knives they can be also carried in different positions like vertical and small of the back carry doesn't cause as much danger as it would with a handgun.
The images from top to bottom: TOPS Baghdad Bullet, unknown Kukri, Boker Plus Schanz Integral Dagger, Emerson La Griffe, Dirk Pinkerton Pocket Bowie, Boker Plus Mosier Tactical, CRKT Crawford Kasper Dragon, and KSF Pocket Sheaths.