I see "Otaku" as meaning anyone who is into anime that does not include Pokemon, Yu-gi-Oh, or Dragon Ball Z. (If you like them, you're not disqualified).
They must be into a wide variety of anime, not just one.
They have to have some understanding of Japanese culture & how it reflects in anime.
Cosplay is not mandatory, but it helps establish the status all the more.
Being able to speak Japanese is not mandatory, but it makes you cooler, even to those who don't like anime.
Finally, you have to be able to convince an anti-anime person that there is more to the artform than Pokemon and porn.