Otaku would be defined as a person entirely obsessed with one specific thing. They are very common in Japan, you can find people conversant in a variety of fields, and because they are so obsessed, they are extremely interesting to talk to, if you have an interest in their field that is.

At the same time, there are people who aren't an Otaku in anything, and it's very common amongst younger females in Japan. This makes them rather boring, they don't seem to have hobbies or seem to be very interested in many things, and means it is very hard to talk to them. You can be an Otaku at cosmetics, shopping, food, anything really, but you should become an Otaku in at least one thing.

Being an Otaku means it's much easier to produce, rather than to only consume. And that's bad. As humans, it is our job to create things. It is our job to be an Otaku.