1. Nail nick - I don't need anything special. Just love that traditional feel and I rarely desire one-handed opening.
2. Thumb stud - My choice in on-handed opening. However, it constantly gets in the way when it's time to freehand sharpen.
3. Flipper - With the flick of the wrist while pressing on the flipper, it's pretty easy to open. That is, as long as the pivot screw isn't too tight and it's well lubed. This isn't always the case, however.
4. Hole - I feel it interferes with my cutting at times and fear that the missing steel might weaken the blade. Also, I have trouble flicking my knives out with this opening method. Don't really know why.
I see no reason why nail nicks aren't added on top of thumb studs or other opening designs. I wouldn't expect it to add too much in terms of the cost of the blade. Perhaps it's simply an aesthetic issue?