my favorite part of the karma is the "fortunes" when you hover over the blue bars. when i first discovered this, my new game was to try to find all the different ones, and i probably spent as much time hovering on people's karma to read the fortune as i did reading the posts : Patricia can load mercury with a pitchfork ! RWood makes transoceanic flights without the assistance of an airplane !! nekokami is a rising star in the heavens !!!
i was very happy the day i discovered my fortune was "zelda_pinwheel plays well with others"
but one of my favorites (so far...) is NatCh's : "NatCh herds cats with both ease and grace." hee !!
but to answer the question, i have been quite touched when people have replied to me after i give them karma, it's a nice attention, so i try to do the same as well, depending (as NatCh said) on whether i notice, whether they leave a comment, etc.