The original meaning of "otaku" has actually a negative connotation, comparable to the stereotypes of Trekkies, Everquest/World of Warcraft/Dungeons and Dragons addicts, and obsessive fangirls (or fanboys) with Jamie Lynn Spears or Johnny Depp all over their bedrooms and clothes. It's pretty much the Japanese word for geek. And people can be "geeky" over lots of things, as you probably know.

To me, an otaku isn't an otaku until the mere sight of anything related to their fandom brings them rapture.