As a rule, for a carry gun, if it hurts to shoot less than 5 consecutive shots, I don't want it. Reason being, I won't train with it, since I won't want to shoot it. That means most ultralight (scandium/titanium) J-Frames, LCP (and similar sized guns), etc are a no go for me. Smallest gun I own is an LCR, and it's actually really nice to shoot. +P is a bit uncomfortable to shoot with it, but not painful.
Beyond that, it depends on why I want the gun, as to whether how painful it is to shoot is a problem. I don't currently own anything that I wouldn't be willing to put 50+ rounds through in a session, with the exception of my Mossberg 500. And even then, light target loads aren't usually a problem.