Like Neil, Blake's 7 is the only thing of which I'd call myself a fan. In my case I think there are two major elements:
(1) the impressionable age at which I first saw the series (under 10)
(2) the believable nastiness of the background, and its relevance to the here and now
What I most like about the series is the characters, but there are plenty of series with similar believably mixed characters, and they haven't drawn me in in the same way.
Why write fanfiction? In my case, purely because I saw good fanfiction on the web last April/May, and thought 'hey, I can do that!' and 'that looks like fun'. Probably nothing to do with the series at all, except that (IMO) it appears to spawn better fanfic, on average, than other series. I've not come up with an explanation for this observation.
Tavia