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. Definition from the urban dictionary. :P