*A few notes: one, why make this a subpage of Star Trek? What other kind of Trekkies have you heard of? two, it's not universally considered derogatory; in fact, some "trekkies" prefer to be called "trekkies," while other fans prefer "trekkers."
It is often considered derogatory, perhaps because of a perceived parallel to the term "groupie". Many Star Trek enthusiasts prefer the term Trekker.
Changed: 5,8c5
Ok, here's the deal. In the beginning all Star Trek fans were Trekkies. Then about the time when Star Trek started having conventions dedicated to it a new class of fan emerged. Instead of just enjoying the show these fans wanted to participate. For instance, these fans are likely to own Star Trek uniforms that they wear to conventions, club meetings and in extreme cases to work. They more likely than not consider Klingon a second language and own English to Klingon dictionaries. It is these super fans that decided Trekkie was demeaning and adopted the term trekker. The casual Star Trek fan doesn't mind being called a Trekkie.