Beyond that, I'm particularly fond of the Delica4, and Endura4 because they accommodate ambi tip up and tip down clip positions.
I'm left handed, but I carry folders on both sides of my body. If a knife can't be set up for carry on either side, I won't buy it.
Tip up or down refers to the orientation of the point when the knife is closed and in your pocket. I prefer tip down for most, but not all, applications. I like to have the option of setting my knives up for either. Carry position is largely a personal preference thing.
...the pivot is adjustable...
The pivot is the point where the blade rotates to open or close. Many knives use a pin (not adjustable) in the pivot. Many others use a screw(adjustable). Pivots loosen over time. It's nice to be able to correct that. Also, with an adjustable pivot, you can select the exact amount of tension that you prefer.
the FRN (fiber reinforced nylon grip scales) versions are very reasonably priced, and have a nice grippy texture
Fiber reinforced nylon is the handle(grip scales) material on the models I'm referring to. There are lots of options when it comes to scales. I like something rough for a good grip.
...the lock system is strong, and doesn't require thin springs...
I don't have pictures to illustrate this, but the lock on the Delica and Endura uses a nice sized hunk of steel as a spring. The Benchmade Axis Lock, or SOG ARG Lock for example, use much thinner springs. More prone to failure, IMO. Probably not an issue for most people. I own several Axis Lock Knives, and none of them have ever failed, but it's something to consider.
...and there are drones available.
These are drones. They're dull, rounded tip versions of certain knives.
If you're going to carry a knife intending to use it as a defensive weapon, you need to train with your knife. Obviously, you can't use a live blade when working with a training partner. Folders in particular require lots of deployment practice. Just as we don't practice drawing loaded pistols, we shouldn't practice drawing and opening live blades.
Most knife companies don't make drones. Spyderco, Benchmade, and Emerson are the only companies that do AFAIK. You could get a live Delica or Endura and
a drone for about the price of one or the other from Benchmade. Less than one or the other from Emerson.
I also prefer Spyderco's RoundHole to thumb studs or discs.
The RoundHole is a hole in the blade that's used to open the knife.
If you visit this link,Cutlery Shoppe - Search - Page 2
, and look around a little, you'll see that all Spyderco folders have them.
Some knives use a stud,Benchmade MINI GRIPTILIAN - Black Handle Plain Edge Drop Point Blade
or a disc Emerson Knives
mounted on, or near the spine of the blade for the same purpose.
I like the hole, but that's really a personal preference issue.
I hope this helps.