Wonderful comparison, Bob! My best friend is a big sports fan, complete with trivial knowledge of decades of sports, fantasy teams in several leagues, wearing jersey's on a regular basis, and even collecting hats from all of his favorite teams.
The same guy hosts board game nights at his house, collects Nintendo games, listens to eclectic musicians like Bjork, and watches foreign and independent films at every opportunity. He covers a wide spectrum of nerdiness, from sports, to music, to games, to movies. Are he, and the others who obsessively follow any of those topics, not nerds by any other name?
Being a nerd means being passionate about something, perhaps to the point of obsession. Whether it's gardening, scrapbooking, literature, music, film, sports, bottle caps and coins, animals, games, or anything else that people like, there are nerds of all kinds.
It's unfortunate that society has stigmatized obsession with only certain topics as being nerdy and that it has equally associated such an atypical, extreme behavior with it. As all us nerds can attest that, though some people do fit the stereotype, there are significantly many more of us who do not. The stereotype is associated with the image, but our obsessions get mixed in with that stereotype. The glasses, acne, squeaky voice, and social ineptitude are themselves a stereotype, independent of being obsessed with any given interest. The "nerd" stereotype has nothing to do with being a nerd.
I've often seen people try to differentiate between the two by discriminating between the term "geek" and "nerd", but the two have been confounded for so long in common speech that it's futile to try. We need a new name.
Incidentally, this awkward stereotype more closely resembles that of pubescence than that of nerdy obsession. Perhaps this is where we can turn to draw the lines between "being obsessed with a subject" and "pimples, glasses, social awkwardness, and unusual clothing styles". A healthy interest in and devotion to a subject does not preclude one from graduating from puberty into adulthood.