The comments about mindset and the like being the most important attribute are true. If I think about things that are truly likely to make a difference in my survivability, mindset, aggression, skill, etc. are at the top.
But is it really true that the choice of knife is so unimportant as to be not worth the discussion? If you imagine yourself to always be starting with your knife in your hand, then perhaps. But in real life, getting the knife in your hand is hard. Anyone who has done real pressure-testing of the draw can attest to this -- if your buddy is charging at you winging punches, getting that knife out is extremely difficult, and a small edge (no pun intended) in deployability can make the difference between "living" or "dying" in the drill's scenario. For me, that dictates the most important features of a defensive folder:
- It must be highly carryable -- not so heavy or thick or long as to be uncomfortable -- so I have one on me
- It must be carryable in a place that's easy for me to grab
- Most importantly, the opening mechanism must, to the highest extent possible, help ensure reliability of opening even when I'm tired and full of adrenaline and my minor muscle control is seriously compromised (e.g., I just fought off an assault empty-handed and now I'm drawing for my life).
Through experiments with my group, a big wave is a true advantage in the above scenario. When everyone is relaxed, thumb stud or hole or wave or whatever doesn't matter. When we raise the stress level in the drill, people start missing studs and (to a lesser extent) holes, but the wave opening works, and even on a blown opening the partially-waved blade can be easily flicked.
Which brings me to my choice of folder: the waved Endura 4. The waved Endura 4 has other important attributes, such as solid construction, great edge geometry, a secure handle, and a very reliable lock. But the consistency of opening makes it the top choice for me.
What I don't think matters much in a purely defensive folder: blade steel, minor changes in blade geometry, handle comfort, etc. However, since most people use their defensive folder as an EDC as well, a good steel and practical blade shape should always be considered.