Common caliber - 9mm/.40S&W/.45acp for autos, .38/.357/.44mag for revolvers
Functional reliability - goes bang when you want it too
Mechanical reliability - does not break often or require professional repair
Spare parts availability - if you can't get parts you can't fix it
User repair-ability - if YOU can't fix it it is useless (see again, spare parts)
Accessories - Magazines, holsters, cleaning kits, lights, whatever doesn't come in the box
The reason I like the 1911 is because there is nothing on the weapon I can't fix short of something requiring a welder. Magazine capacity is less important to me because if it is a "Combat" pistol it means it is a back up to my primary weapon (rifle, shotgun, etc.) while if it is a concealment weapon I am more concerned with keeping it hidden. I do like the most BB's I can get, but I won't sacrifice the rest for a few extra rounds. I don't like revolvers simply because they are almost impossible to fix without considerable training and ability, to say nothing about specialized tools.
Also, too many people forget that a gun, 2 mags and a box of ammo does not constitute being armed. Any weapon is a system and you need a complete system if you want to be armed, if you have no holster it is hard to carry, if you have no way to clean it properly it will stop working at some point.
Lastly, too many people think "Tests show that my _______ will work for 100,000 rounds", and it might, but it might not. It might never break, but everything made by humans can and will break at some point, if you want to bet you life on your (YOUR, not the one in the tests) pistol never failing, go ahead, but I won't. Better to have the parts and the skills to use them then end up unarmed because the PR from the people who want to make money turns out to be a tiny bit incorrect.