this actually recently came up in a discussion with some people in my Kendo class and the Japanese meaning of the word is very different then how Americans tend to use the term.
the following definition will explain why some people oppose the term. (its the same as the one from the people in my Kendo class who know more than me)
as taken from urbandictionary.com link
Quote
1. otaku
Otaku is the honorific word of Taku (home).
Otaku is extremely negative in meaning as it is used to refer to someone who stays at home all the time and doesn't have a life (no social life, no love life, etc)
Usually an otaku person has nothing better to do with their life so they pass the time by watching anime, playing videogames, surfing the internet (otaku is also used to refer to a nerd/hacker/programmer).
In the Western culture, people confuse otaku to be something positive like "Guru". If you think about it, it's not really good to be called a guru if it means you are a total loser who can't socialize with other people except through the Internet.
Other Japanese words which have been confused by Westerners also include but not limited to: Anime, Manga, etc
-Darth Simon