1) If I don't remember who you are, hide. 2) If I knew you in elementary/middle/high school/college and I don't think I'll ever see or speak to you again, hide. (This would cover people who don't share any of my interests, etc). 3) If you spam my newsfeed with "Farmville/Which Twilight character are you/I have a secret to tell you" sort of apps, hide. 4) If we have nothing in common, and I don't think anything you're going to say in the future is going to be interesting/have a real impact on me, hide.
There may be one or two more, but those were the core ones. After applying these rules, I found Facebook was SO much nicer and more fun to use, and there was none of the social stigma of "unfriending" people.