Rule #1: Anyone who adds me gets added back, by default. That's automatic, even if I think the person's a complete douchebag. They added me, they can't be all bad.
Rule #2: Anyone who has a lively discussion with me in a message board thread. You create a quote chain with me in dire need of a snip, you get friended.
Rule #3: Anyone who I think "if I ever get a chance to meet this person in real life, I'm checking my vacation days and booking a flight." Note that Rule #3 is usually satisfied by Rule #2, but not always. Sometimes a bunch of little comments beat one big chain of 'em.