Reliable- Obviously, without a gun you can trust to work under most all reasonable conditions, there is no gun to evaluate for carrying. So of course we want the gun to be reliable with hollow points, in adverse conditions, etc.
Grip- The texture has to be rough and have enough traction for my hand to stay on in all conditions, mostly due to me getting very sweaty and grimy hands. It also must allow a point of aim that does not require me to dip my hand forward like you have to do with a Glock. This is what has led to great results with me under stress. The last thing I set priority on is if the mag is flush with the magwell or if it is extended to help with the grip. I do not need extensions, so that could be a deal-breaker since I want the pistol to be as low in overall height as possible.
Trigger- My requirement for this is not necessarily restrictive on the trigger type, but more on its' tolerance to my technique and ability to manipulate it to great affect. For me, travel is the only limiting factor, as well as weight, but both to a point. I do not believe in training to use the trigger reset as the point at which to start followup shots, as I have found it to be a detail bypassed when you really have your life in your own hands. Again this is based on experience and not some tacticool Hollywood-esque daydream. I do have limitations on travel and weight though. I have no issue with DAO triggers as long as the full stroke is not beyond my reasonable reach of these triggers at their reset point, which would be at the full start position of the trigger.
With these specific qualifications, the rest of the features about the pistol is negotiable. I don't really put a priority on the sights, controls, mag capacity, accessories, or even the disassembly. We all have to come up with our own standards on what we can handle and live with. I have spent alot of time finding out what those priorities are for my pistols.
With all this said, I have found the Walther CCP to be the best all around carry pistol in my hands. This pistol draws alot of criticism from the market for having a safety, having only 8 rounds, having a different system and disassembly, and other things that are trivial at best. But all in all, I perform best with this pistol, and I have found that the grip and the trigger to be the reason, giving me the revelation that those two factors are the most important for me. I can work out the other kinks.
What are your deal-breakers?