Reliability first, invisibility second.
I agree 100% with those who say that reliability is the first prerequisite of the ideal concealed SD weapon.
But the gun's mechanical reliability is only the starting point.
I would extend the term "reliability" to include all aspects of handling the weapon: shooting it, the ease of carrying it, speed of presentation, use of proven ammunition, and so on. These are all factors that contribute to making the weapon "reliable" as far as the user is concerned. (To cite an example, nunchucks are considered a deadly weapon in certain jurisdictions, but are a "reliable" recourse only to those accustomed to using them. A gun in the hands of some one who can hardly shoot it can hardly be considered a "reliable" weapon. It might go bang every time, but can it really safe him/her when a life is on the line?)
In this vein, a "reliable" handgun would be one in an acceptable defense caliber, loaded with police-certified JHP ammo, which you are intimately familiar with, and regularly train with. You must be able to carry it, draw it from concealment quickly, and consistently hit with it.
The second requisite, "invisibility", should be self-explanatory in ccw parlance.