In 2010 a psychology graduate student did one or two surveys on motivation for people using, and especially answering, on MO, see http://tea.mathoverflow.net/discussion/890/mathoverflow-survey-update/ Tausczik eventually published her results in psychology journals. She had been an undergraduate math major at Berkeley when MO was still in its formative stages, so she had an early interest.
I was one of the ones telephoned. At the time, I gave a lot of high-flown reasons, helping people, what have you. I think that is how many people feel for the first year or two of contributing on a site of this type. For me, there was a middle era where i mostly felt i was just showing off what i knew (this is more MSE, really), finally a relaxed time where I just answer when I feel like it. Similar with MSE, just a delay of a year. Things being what they are, it appears that those who go full blast answering questions for years at a furious pace are fairly likely to quit completely when that becomes tiresome.
Anyway, the sites are not parallel in one way that is significant for the question. On MO it is peers helping peers. On MSE it is more one-sided, more strictly teaching, and motivation does differ. I suppose I have mixed together descriptions of my behavior on the two sites. Oh, well.
It occurs to me that I can be quite specific about reputation. On MO, before it joined stackexchange proper, it was possible for a 10K user to search deleted posts, including questions deleted by the person asking, all the way back to the beginning of the site. I did this in connection with this unpleasant episode: http://tea.mathoverflow.net/discussion/1187/extending-from-a-plane-in-r3-again-and-again-and-again/ Under the circumstances, I felt it was desirable to quickly get up to 10K here on MSE, but was disappointed to find that I had severely limited ability to search deleted posts, and no way at all to see self-deleted questions. Also there was something about not being 100% trusted until 20K. So I did that, and found no improvement in searching. After that, I felt that I had no specific benefit in answering tons of questions that I did not necessarily enjoy, or tinkering with adequate answers so as to get more points, so I just slowed down.