Regular job, five years of constant residence, no criminal record, fluency in Japanese, willingness to take a Japanese name... Basically the usual stuff to acquire citizenship for a country. You have to give up your citizenship back home, though, so unless you're really set on living in Japan forever, it's probably not worth it. Especially since you don't really gain that much compared to someone with "permanent resident" visa status, besides the ability to vote and run for parliament, and probably a little more security in terms of your ability to stay in the country.
Still, I thought it was cute that they suggested it.

It's nice to know I'm so accepted here that my friends want to make me one of them. :'D