Great question! With the potential to get really philosophical, and I like that. :-)
In my opinion, the "keeper" has to be the shot that has captured the moment, and, like @gatorguy points out, the technical "perfection" of the image is secondary to the spirit of the image. If it's a little off-square, or a little out-of-focus, but the moment is there, then it's kept. It's harder for me to justify keeping an image where the exposure (light) is completely wrong, but thankfully I shoot everything in RAW and a lot of those files can be saved.
The longer I shoot (and I've only had DSLRs since 2013), the more cut throat I am about the images I take and make. But because I'm being more careful about getting the shot in the first place, I have less work to do when I upload the images into Lightroom and start culling. There are times when I'm out for a walk with my camera and see something that might be worth firing the shutter... but once I look at it through the viewfinder and do some more careful thinking, I skip it.
I also think it's a good idea to take multiple shots of the same subject. Look at it, walk around it, see it from a different angle, in a different light - the subject caught your eye, but you ought to explore it and make sure you tease out whatever it may be hiding, something subtle that you didn't notice at first glance... Don't just point and snap and walk away... This will increase the number of shots you take, and decrease your "keeper" rate, but maybe you'll end up being pleasantly surprised by the exercise...
Mathematically, with all those things considered, my personal keeper rate is about 10% on average. If I take 100 shots, I end up polishing and publishing maybe 10. (This rate doesn't apply to family snaps, where I pretty much keep everything.)