Any post that I like, basically. If it makes me laugh out loud, or smile a secret smile; if it says something eloquently and well; if it's original, thoughtful, but also if the poster shows impressive restraint, I rep for that too. Like if someone makes a post and someone else replies in an inflammatory way, but the other person stays cool and calm and doesn't descend to insults etc, I rep for that too.
I haven't neg repped except once and I wish I hadn't done that either really. Reason being that you never know who will take it personally or the wrong way or whatever, and I can't be bothered with people harbouring grudges against me for silly things like that. If someone's gonna have a grudge I'd rather it was for something that I meant to do.
If I did neg rep, then it'd be for posts containing personal insults (not just to me, but anyone), hypocrisy, BS intellectual conceits, received wisdom, and general non-coolness. Again, because I'm also sometimes guilty of these things, I don't really wanna 'cast stones'...
I also don't give rep points to people who represent serious competition for me, for the top spot *
I hemmed and hawed over whether to hide my green boxes, but opted to show them in the end, because I figured to hide them would be hypocrisy on my part, pretending a modesty that isn't truly sincere. I'd rather be sincerely vain than falsely modest. To me, insincerity is one of the worst crimes.
edit - I don't give them for 'friendly' or 'bonding' purposes, and I can't decide whether it irritates me or not, when people give them to me for those reasons. I guess it would, if I didn't actually appreciate the sentiment - only sorta half-wish it'd be expressed some other way (I say half, because I am vain and competitive enough to just think 'yay! more points!' as well). I sorta want my rep to mean something... something more than just 'someone who dishes out a lot of points per rep likes me a lot'. But as I say, I do appreciate the motive and sentiment, so I don't actively try to discourage it for fear of hurting the feelings of the people who do it, only meaning well. I don't want them to feel like I'm rejecting them or their intentions, cos I don't. And I also don't want them to stop doing it, or I'll never beat PinkPiranha or whatever to the top spot!!