As a general rule, I look at a good utility knife as a poor defensive weapon. There are some exceptions such as the Spyderco Chinook series, but as a general rule the two knives should be different. One major reason is because a utility knife will at certain points in time be dull. Utility knives are designed for endurance and they are often thick. A 'people' knife can be almost paper thin to aid it in cutting. Skin and clothes don't require a thick blade. In fact, a thick blade can make usage more difficult IMO. My first qualification would be a thin blade. My second qualification would be the blade have deep serrations with the reason being is that you need something that eats through thick clothes to reach skin. 'Teeth' easy go through even leather jackets.
I also look for a knife with a lower hardness level and with a steel that is considered 'tough'...that is, metal properties favor impact resistance over hardness. High chromium steels are nice too (among a few other mixes) to prevent the blade from corroding.
I also look for a curved blade which I feel is by far the best choice. When I say curved, I am referring to a hawk billed or reverse S curve blade. More specifically, I am referring to the Spyderco Civilian, Spyderco Matriarch, and Spyderco Lil' Matriarch which IMO as the finest defense knives on the market.
I look to hawkbills because of their tendency to cut deeper as you move. If you catch a wrist, arm, or leg with a hawkbill, when the assailant tries to pull their wrist back, the hawkbill tip by nature buries itself deeper and deeper which means a large and deep tearing cut. Rather than a small puncture 'jab', you now have a very deep wound and the clock is ticking until the assailant is down. The other reason I prefer the Reverse S Curve is because you can 'peck' with it. You can peck at the eyes and easily make contact. You can also make controlled cuts which will sever entire muscles essentially ending the encounter. With how it cuts, targeting these muscles is a very easy task.
The other criteria I look for is a non-'thrusting' knife because a stab as such makes the user vulnerable, especially certain vitals. A hawkbill held in a reverse hammer grip can be used as a shield as well. Simply bring the knife into defending vitals with the business end facing outward, and when the assailant strikes, let the knife catch him. The amount of work done with it is far less.
Finally, I look for a knife that is intimidating. I believe the Spyderco Civilian is probably the most intimidating folding knife on the planet.
...but I would argue that this: (full size Civilian with 4.2 inch blade)
or this: (mid size Matriarch--there is also a mini version of this)
+ James Keating's 'The Spyderco Civilian Video' cannot be matched by any other folding knife. Of course all of this is just my opinion.
