So far, with the fact that I have plenty of space at my disposal in mind, I only throw away the shots which are technically flawed. That is to say that they are ruined by poor focus, wrong exposure or motion blur and not for their lack of artistic or aesthetic appeal.
I tend to keep the so-so shots anyway. I find that some shots that look average or even awful can turn out quite well when post-processed(quite heavily). Sometimes when I go back and look at the images I've taken, I think of new ideas for it that I may not have thought of before so it is for this reason that I keep most of my images.
I shot in JPG originally but now shoot only in RAW and I'm still trying to find out if the merits of RAW are worth the extra space and the possible lack of future compatibility given that all of my images are for personal use only at the moment.
I've only been got 18+GB of images at the moment, I've only been taking photography seriously for about half a year. Who knows? I might adopt a more strict criteria when space really becomes an issue!
...and by "issue", I'm talking terabytes...